Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.
CAKE: Combine buttermilk, corn syrup and lemon juice in a small bowl. Set aside. Cream butter and sugar in a large mixer bowl until light and fluffy. Beat in eggs, one at a time, then vanilla and lemon peel. Sift flour, corn starch, baking soda, baking powder and salt together. Add to butter mixture alternately with buttermilk mixture, mixing on low speed after each addition until combined. Spread batter evenly in prepared pans.
Bake 20 to 25 minutes until toothpick inserted in center comes out clean. Cool in pan 15 minutes. Remove to wire rack. Cool completely. While cake is baking and cooling, prepare Lemon Filling and Lemon Buttercream Frosting.
LEMON FILLING: Combine sugar, corn starch and salt in a medium saucepan. Stir in water until smooth. Bring to a simmer over medium heat, stirring frequently. When mixture starts to simmer and turn translucent, whisk in egg yolks, then lemon peel, lemon juice and butter. Bring mixture to a full simmer, stirring constantly. Remove from heat, and transfer to another container to cool. Press a piece of plastic wrap to surface of filling and cool to room temperature. Do not stir once filling is set. (Filling may also be refrigerated overnight.)
LEMON BUTTERCREAM FROSTING: Combine all frosting ingredients in a large mixer bowl. Beat until light and fluffy. Add additional milk or lemon juice, 1 teaspoon at a time, as needed, to reach desired spreading consistency.
Spread Lemon Filling between cake layers. Frost top and sides of cake with Lemon Buttercream Frosting. Chill 2 hours or until set.