Congratulations if you made it this far!! You're a rock star! Now relax and enjoy the process of putting together a delicious cake! Remove the cake from the freezer to thaw if it is frozen. If the buttercream has been in the refrigerator, allow it to set at room temperature for a few hours before remixing the buttercream and assembly.
Meanwhile, to remix the buttercream, fit a stand mixer with a paddle attachment. Place all the buttercream into the stand mixer and mix on low to get most, if not all of the air bubbles out. While mixing to reduce the air bubbles, add 12 Tbs of lemon curd, one at a time, to the buttercream. Mix for about 5 minutes, on low, or longer to achieve the desired consistency.
Using a serrated knife, slice each 1/2 of cake into two equal rounds. Place the cake on a revolving cake stand with one hand on top of the cake while the other hand begins to cut with a serrated knife. Put a bit of pressure on the cake to turn the cake stand while the serrated knife begins to cut. Let the knife do the work. Be aware of the knife tip and make sure it doesn't float upwards while cutting as it should be kept level the entire time. Cut the two cake rounds into 4 rounds. Set aside.
Take the top of one of the rounds and place it upside down on the revolving cake stand. Remove the parchment. Brush the top with the lemon simple syrup, covering the entire top. I do this twice for each layer.
Fill a pasty bag fitted with round tip #806 with buttercream. Pipe a boarder/barrier around the top edge of the layer. Spoon 3 Tbs of lemon curd in the center and spread to the barrier. Pipe an even layer of buttercream in a circular motion over the lemon curd. Use an offset spatula to smooth out.
Repeat this procedure until the top layer of the cake is placed on, saving a bottom piece for the top layer.
Place the top layer on. This should be an inverted bottom 1/2 of one of the cake rounds. After placing the top layer of cake, put your hand, flat, on the top of the cake. Press gently, yet firmly, down making sure the cake is symmetrical and the layers are aligned. Make adjustments now if needed.
Brush two coats of lemon simple syrup over the top layer. You are now ready for the crumb coat which is a thin base layer of buttercream that is the foundation of the final coat of buttercream. Have a spare 'dump' bowl ready to clean the offset spatula off. The buttercream that is being cleaned off in the dump bowl will have crumbs in it. This is not something you want mixed in with the 'clean' buttercream. So keep it separated!
Place about 1/2 C buttercream on top on the cake. Begin working the buttercream by spinning the cake stand and using an offset spatula to smooth and move the buttercream over the sides of the cake. Wipe the offset spatula in the dump bowl as needed. Spin the cake stand as you create a smooth, thin finish, covering all areas of the cake. If more buttercream is needed, use it.
Using your largest offset spatula, flat side down, 'cut' the buttercream off the top edge, gently, while moving the spatula towards the center of the cake then lifting the spatula off the cake like an airplane taking off. Wipe the spatula clean. Spin the cake stand to the next section and repeat until all edges are clean and neat. Repeat if needed.
Place the crumb coated cake in the refrigerator for at least 30 minutes.
For the final coat, using about 1 C of buttercream, start smoothing out the top of the cake by rotating the cake stand and using a pallet knife to push the buttercream over the edge. Smooth out the top. Use a pallet knife to smooth the buttercream around all sides of the cake, stopping to patch areas that need more. While you're doing this, you'll notice that the top edges have buttercream taller than the top of the cake. This is fine. We'll 'cut' that buttercream off shortly. The focus now is to get the sides as smooth as possible. Use a bench scraper held perpendicular to the cake stand, and 45 degrees to the cake and rotate the cake stand, smoothing out the sides. When you're satisfied, move on to the top. If you find there are still some air bubbles in the icing, fill a tall glass with hot water, dip the offset spatula in the water, wipe dry and smooth over the cake. The warmth of the spatula helps smooth the bubbles.
Using your largest offset spatula, flat side down, 'cut' the buttercream off the edge, gently, while moving the spatula towards the center of the cake then lifting the spatula off the cake like an airplane taking off. Wipe the spatula clean. Spin the cake stand to the next section and repeat until all edges are clean and neat. Repeat if needed.
Use an offset spatula to clean the edges around the base of the cake by placing the spatula at a slight angle with just the tip of the spatula under the edge of the buttercream yet still touching the revolving cake stand. Spin the cake stand to clean the edge. This will be covered with piping. Refrigerate for at least 30 minutes before transferring to the serving stand.
Transfer the cake to a cake stand by sliding a large offset spatula under the cake. Very gently, lift the cake and place the cake on the cake stand. Gently slide the offset spatula out from under the cake, pressing the offset spatula down so the tip is pressing against the cake stand rather than the cake. Gently slide the offset spatula out. Patch up any areas that may have suffered a bit of damage.
Fit a piping bag with the open star tip #18 and fill it with buttercream. Practice piping shells on a piece of parchment first, then on the cake. Place your cake that is on the decorative cake stand on the rotating cake stand so that you can spin the cake as you pipe. For the base, hold the piping bag at a 45 degree angle to the cake, and hover the tip just over the cake. Apply steady, even pressure to the bag pushing away from your body slightly, then pulling the bag up and down towards you. Release the pressure as you pull down. Start the next shell the same way, at the base of the shell you just created.
For the top of the cake, hold the bag perpendicular to the cake and pipe as indicated above. Finish the base and the top of the cake all the way around. If you find that you just can't stand one or two of the shells you piped, use an offset spatula to gently scoop it off and repipe. Place the cake in the refrigerator for about 30 minutes before placing sugar flowers or other decoration on top.
Place in the refrigerator.