Cake

• Mix the cake batter according to the recipe, adding the finely grated zest of 1 lemon to the dry ingredients and 2 teaspoons lemon extract with the vanilla.
• Divide the batter between two 8"x3" 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).
• 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.

Have your syrup, buttercream and lemon curd ready before you begin making the Lemon Mousse.

Lemon Mousse

• Place 1 ½ oz Limoncello (or cold water) and 1 teaspoon lemon extract into a microwave safe bowl. Sprinkle 1 ½ teaspoons gelatin powder over the liquid in an even layer. Whisk briefly to combine and set aside to bloom. Whip 8 oz heavy cream with 2 tablespoons powdered sugar and set aside.
• Place the 12 oz Lemon Curd (1¼ cups) in a large mixing bowl and use a spatula to smooth out the curd to break up any lumps. Fold ⅓ of the whipped cream into the lemon curd until no lumps of curd remain. Fold in the remaining cream until it's about ½ way incorporated.
• Heat the bloomed gelatin in the microwave in 10 second increments until it's hot to the touch. Working quickly, add a ½ 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.

Assembly

• Fill a piping bag fitted with a star tip with buttercream (or use a disposable bag with the tip cut).
• 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.
• 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 ½ 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).
• 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.
• 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.
• Use a piping bag fitted with a star tip to pipe a border around the top of the cake. Spread the last ½ cup of curd onto the cake. Refrigerate for at least 4 hours to make sure the filling is completely set, preferably over night.
• Remove the cake from the refrigerator ½ hour before serving.