Lemon Layer Cake with Tart Lemon Curd and Lemon Buttercream

From 6 Bittersweets Guest Recipe
Cake layers adapted from Ina Garten
Makes one 3-layer 9-inch round cake

XIAOLU'S NOTES: I recommend making the lemon curd ahead of time so that it has time to cool to room temperature.

1/4 cup freshly grated lemon zest (4 to 5 large lemons)
2 1/3 cups sugar, divided
1 cup (2 sticks) unsalted butter, room temperature
4 large eggs PLUS 1 large yolk, room temperature
2 cups (9 oz.) all purpose flour
1 cup (4 oz.) cake flour
1/2 tsp baking powder
1/2 tsp baking soda
3/4 tsp salt
1/4 cup PLUS 1/3 cup freshly squeezed lemon juice (from 3 to 4 lemons), divided
3/4 cup buttermilk, room temperature
1 tsp pure vanilla extract
Lemon Cream Cheese Crusting Buttercream (Recipe below)
Lemon Curd Filling (Recipe below)
Finely and freshly grated zest of 2 to 3 large lemons, to garnish

Preheat the oven to 350 degrees F. Line the bottoms of three 9-inch round pans with parchment paper circles, then butter and flour the pans.

Combine the lemon zest and 2 cups of sugar in a large mixing bowl, and use your fingers to rub them together until sugar is fragrant. Add butter to the sugar mixture, then mix with an electric mixer on medium speed until light and fluffy, about 5 minutes. Add the eggs and yolk, one at a time, until combined.

Sift together the flours, baking powder, baking soda, and salt into a bowl. In another bowl, combine 1/4 cup lemon juice, buttermilk, and vanilla. Add 1/3 of the flour mixture to the creamed butter mixture and mix on medium speed until just combined. Next, add 1/3 the flour mixture, then 1/2 the milk mixture, another 1/3 of the flour mixture, the remaining milk mixture, and the last 1/3 of the flour mixture, mixing to combine after each addition.

Divide the batter evenly between the prepared pans, smooth the tops with a spatula or spoon, and bake in the center of the oven 22 to 28 minutes, or until a wooden toothpick inserted in the center comes out clean. Cool the layers in pans on wire racks for 20 minutes, then turn them out onto racks to cool completely.

In the meantime, combine the remaining 1/3 cup of sugar with 1/3 cup of lemon juice in a small saucepan and cook over low heat until the sugar dissolves. Once cakes are completely cooled, level cake layers with a serrated knife, if uneven. Brush off any crumbs that stick to the outside of the cakes.

To assemble the cake, place one cake layer on a cake plate. Brush lemon syrup over entire top surface of the cake. Transfer some buttercream to a piping bag fitted with a medium round piping tip, and pipe a circle of frosting around the edge of the first layer of cake (to act as a dam to stop lemon curd from squeezing out of the sides of the cake later). Spoon 1/2 to 2/3 of a cup of lemon curd onto the middle of the cake layer and spread evenly until curd meets the frosting at the edges. Place a second cake layer on top of the first layer, and brush with syrup and apply buttercream and the remaining lemon curd in the same way as above. Finally, place the third cake layer on top, then use a pastry brush to remove any remaining crumbs from the sides of the cake. Brush top of final layer with lemon syrup.

Frost the cake as desired with the remaining buttercream, covering both top and sides. You can use a spoon or spatula to make more rustic waves or you can use a bench scraper to smooth the edges as I have. Sprinkle freshly grated lemon zest over the top of the cake shortly before serving.

Lemon Cream Cheese Crusting Buttercream
Adapted from Cake Central

5 oz. cream cheese, cool room temperature
1 cup (2 sticks) unsalted butter, room temperature
1 1/2 tsp freshly grated lemon zest
2 to 3 Tbsp freshly squeezed lemon juice
1 tsp pure vanilla extract
Large pinch of salt
1 lb. 5 oz. powdered sugar, sifted

Beat cream cheese, butter, lemon zest, half of lemon juice, and vanilla with an electric mixer on medium speed until creamy. Scrape down sides of bowl and beater at least once and beat again. Add salt and half of powdered sugar and beat on medium-low speed. Scrape down sides and beater. Add remaining lemon juice and continue beating on medium-low until well incorporated. Scrape down sides and beater again. Add remaining powdered sugar and beat until incorporated. Scrape down sides and beater once more, and then beat on medium-low speed 7 to 10 minutes until completely smooth and spreadable. Use right away.

Lemon Curd
Makes 1 to 1 1/2 cups

3/4 cup sugar
2 tsp cornstarch, sifted
1 large egg
3 large yolks
1/2 cup strained, fresh lemon juice (from 3 lemons)
2 tsp finely grated lemon zest (from 1 lemon)
6 Tbsp unsalted butter, cut into 4 pieces
1/2 tsp pure vanilla extract

Combine sugar and cornstarch until evenly combined in a small to medium saucepan. Add the egg and yolks to the sugar mixture and whisk until evenly mixed. Then add the lemon juice and zest and whisk well.

Heat the saucepan over medium heat while whisking constantly. Continue cooking until the mixture thickens and begins to boil. Let boil for about 1 minute, still whisking constantly, then remove from heat and strain the curd through the sieve into a bowl. Stir in butter chunks and vanilla and whisk until completely incorporated (there should be no chunks of butter left). Let cool completely before using. If not using right after cooling, transfer to an airtight container and store in the fridge until ready to use.