Step 1

Mix the cake according to the recipe, adding the zest to the dry ingredients and the lemon extract with the vanilla.

Step 2

Divide the batter between two 8" pans lined with a parchment round or buttered and floured. Bake as instructed in the recipe. Wrap the cooled layers and chill until firm (I like to bake the cake a day ahead)

Step 3

When you're ready to assemble the cake, trim the browned edges and domed top off both cakes. Split each cake horizontally so you have a total of 4 cake layers. See how to prep the layers here. Have your syrup and buttercream ready before you begin making the Lemon Mousse.

Step 4

Place the Limoncello (or cold water) and lemon extract into a microwave safe bowl. Sprinkle the gelatin over the liquid in an even layer. Whisk briefly to combine and set aside to bloom. Whip the cream with the powdered sugar and set aside.

Step 5

Place the 1 1/4 cups lemon curd in a large mixing bowl and use a spatula to smooth out the curd to break up any lumps. Fold 1/3 of the the cream into the lemon curd until no lumps of curd remain. Fold in the remaining cream until it's about 1/2 way incorporated.

Step 6

Heat the bloomed gelatin in the microwave in 10 second increments until it's hot to the touch. Working quickly, add a 1/2 cup of the mousse to the warmed gelatin. Whisk immediately until completely incorporated. Pour the gelatin mixture back into the mousse. Immediately whisk until the mousse is smooth and the gelatin is evenly incorporated. If the mousse is very soft, refrigerate briefly to let it thicken before assembling the cake.

Step 7

Fill a piping bag fitted with a star tip with buttercream (or use a disposable bag with the tip cut).

Step 8

Set one layer of the cake, flat side down, onto a cardboard cake round or a serving plate. Brush the layer generously with the Limoncello syrup. Pipe a ring of buttercream around the edge of the layer to form a "dam" for the mousse. Scoop half the lemon mousse onto the layer and smooth until even.

Step 9

Gently place the second cake layer onto the mousse, careful not to squish the mousse layer. Brush the layer with syrup and pipe a dam of buttercream. Set aside a 1/2 cup of the lemon curd and scoop the rest onto the layer. Smooth the curd until even (be careful not too press hard or you might squish the mousse layer beneath).

Step 10

Place the 3rd cake layer, brush with syrup and pipe a dam. Fill with the remaining lemon mousse. Top with the 4th layer, brush with syrup. Set the cake into the refrigerator for about 20 minutes to set the buttercream "dams" and the mousse.

Step 11

Ice the cake with a thin crumb coat of buttercream. Refrigerate the cake for at least an hour to set the buttercream and allow the mousse time to gel. Ice the cake with a final coating of buttercream.

Step 12

Use a piping bag fitted with a star tip to pipe a border around the top of the cake. Spread the last 1/2 cup of curd onto the cake. Refrigerate for at least 3-4 hours to make sure the filling is completely set, preferably over night.

Step 13

Remove the cake from the refrigerator 1/2 hour before serving.