How to Make It
Prepare the Cake Layers: Preheat oven to 350°F. Whisk together flour, baking powder, and salt in a medium bowl.
Beat butter and sugar with an electric mixer on medium speed until light and fluffy, about 5 minutes. Add eggs and egg yolks, 1 at a time, beating well after each addition. Beat in vanilla.
Add flour mixture to egg mixture alternately with milk in 5 additions, beginning and ending with flour mixture. Beat on low speed until just combined after each addition. Divide batter evenly among 3 greased and floured 9-inch round cake pans.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 23 to 25 minutes. Cool in pans on wire racks 10 minutes. Remove Cake Layers from pans to wire racks; cool completely, about 30 minutes. When cool, gently slice each layer in half horizontally, making 6 thin layers.
Prepare the Buttercream Frostings: Beat butter with an electric mixer on medium speed until creamy, about 3 minutes. With mixer running on low speed, gradually add powdered sugar, and beat until smooth. Add vanilla, salt, and 3 tablespoons of the milk; beat until smooth, about 1 minute. (Add up to 1 tablespoon more milk, if needed to reach desired consistency.) Remove and reserve 1 cup of frosting in a small bowl. Divide remaining frosting evenly between 2 medium bowls.
Stir lemon zest and juice into 1 medium bowl of frosting. Stir in yellow food coloring to achieve desired color.
Add cocoa to remaining medium bowl of frosting, and stir until smooth. Place chocolate and remaining 1 tablespoon milk in a small microwavable bowl. Microwave on HIGH until chocolate melts and mixture is smooth, about 20 seconds, stirring every 10 seconds. Cool 5 minutes, and stir into chocolate frosting.
Place 1 Cake Layer on a serving plate. Spread about 1⁄3 cup lemon curd on half of layer. Spread about 1⁄3 cup chocolate pudding on the opposite half. Repeat process with 4 more Cake Layers. Top with remaining layer. (Indicate on a sticky note which flavor is on each side, and attach to serving plate.) Spread 1 cup reserved plain frosting in a thin layer over sides. Chill 1 hour.
Spread lemon frosting over top and frosted sides of lemon half of cake. Spread chocolate frosting over top and frosted sides of chocolate half of cake.