Lemon Layer Cake with Lemon Curd

Beat sugar and butter for curd in a bowl with an electric mixer until light and fluffy. Beat in eggs and yolks slowly until well combined. Beat for 1 minute more; stir in lemon juice. The mixture will look curdled; do not panic.

Pour mixture into a saucepan and cook over low heat until it becomes smooth, about 5 minutes. Increase heat to medium and cook, stirring constantly without letting it boil, until it thickens enough to leave a path on the back of a spoon when you drag a finger through it and reaches 170 degrees F (77 degrees C), about 5 minutes. Remove from heat and stir in lemon zest.

Let cool to room temperature, 20 to 30 minutes. Press plastic wrap on the surface to prevent a skin from forming and chill in the refrigerator.

While filling cools, place a rack in the middle of the oven and preheat to 350 degrees F (175 degrees C). Butter and flour two 8- or 9-inch round cake pans.

Sift cake flour, baking powder, and salt together in a medium bowl. Pulse 1/4 cup sugar and lemon zest together in a food processor until well combined.

Beat butter and lemon sugar in a large bowl with an electric mixer on medium speed until light and fluffy, about 1 1/2 minutes. Add remaining sugar and beat until smooth, about 1 1/2 minutes. Beat in 1/4 of the milk until just blended. Add flour mixture alternately with remaining milk on low speed in 3 batches, scraping the bowl with a rubber spatula as needed; beat until just blended.

Beat egg whites in another bowl on medium speed just until foamy. Add cream of tartar. Increase speed to medium-high and beat until stiff peaks form. Add 1/4 of the egg whites to the cake batter and gently fold in. Continue to gently fold in egg whites, being careful not to deflate the mixture. Divide batter between the prepared pans.

Bake in the preheated oven until a toothpick inserted into the center of each cake layer comes out clean, 35 to 40 minutes. Flip right side-up and let cool on a wire rack, 30 minutes to 1 hour.

Cut cooled layers in half and layer with curd.

Beat butter and lemon zest for frosting in a medium bowl until light and fluffy. Add confectioners' sugar and beat. Add lemon juice and beat for 1 minute. Frost the assembled cake as desired.