Ingredients
For cake
2 1/3 cups cake flour
2-3/4 tsp baking powder
1/4 tsp salt
1 3/4 cups granulated sugar
2 tbsp finely grated lemon zest
6 oz unsalted butter, at room temperature
1 cup milk, at room temperature
5 large egg whites, at room temperature
1/4 tsp cream of tartar
For lemon curd
2 tsp finely grated fresh lemon zest
1/4 cup fresh lemon juice
1/4 cup and 2 tbsp granulated sugar
3 large egg yolks
1/4 tsp guar gum
2 oz unsalted butter
1/4 tsp lemon extract
For lemon frosting
8 oz unsalted butter, softened
3 1/2 cups confectioners sugar
1/4 cup fresh lemon juice
2 tsp finely grated fresh lemon zest
1/2 tsp lemon extract
Directions
Cake
1. Position a rack in the middle of the oven; heat the oven to 350 F. Butter and flour two 8 x 2-inch round cake pans. In a medium bowl, sift the cake flour, baking powder, and salt. Pulse 1/4 cup of the sugar with the lemon zest in a food processor until well combined.
2. In a large bowl, beat the butter and lemon sugar in an electric mixer on medium speed until light and fluffy. Add the remaining sugar and beat until smooth for about 1 minute. Pour in a quarter of the milk until blended. On low speed, add the flour mixture with the milk in three batches, scraping the bowl with a rubber spatula; beat just until blended.
3. In another bowl, beat the egg whites with an electric mixer on medium speed until foamy. Add the cream of tartar on medium speed; beat just until the whites form stiff peaks. Add a quarter of the whites to the batter; carefully fold them in with a whisk or a rubber spatula; continue to gently fold in the whites, a quarter at a time.
4. Divide the batter evenly between the prepared pans. Smooth the tops with the spatula. Bake for 35-40 minutes. Cool in the pans on a rack for 10 minutes.
Lemon curd
1. Whisk together lemon zest, juice, sugar, egg yolks, guar gum, and a pinch of salt in a 1-quart heavy saucepan.
2. Add butter and cook over low heat, constantly stirring, until curd is thick enough to hold marks of whisk and first bubbles appear on surface.
3. Whisk in extract. Immediately pour curd into a bowl, then cover surface with wax paper and chill until cold, about 30 minutes.
Make frosting
1. Beat butter with an electric mixer at high speed until light and fluffy, about 1 minute.
2. On the low speed, add confectioners sugar, lemon juice, extract, and zest. Mix until creamy and smooth, about 2 minutes.
Frost cake
1. Cut the cake in half horizontally, using a long serrated knife. Put one of the four cake layers on a serving plate, cut side up.
2. With an offset spatula or a table knife, spread a 1/3 cup chilled lemon curd on top of the cake layer.
3. Repeat with the second and third cake layer. Top with the fourth cake layer.
