For the Cake: Preheat oven to 350 degrees. Cream together the butter, sugar, and lemon zest until smooth and creamy. Sift together the flour, baking powder and salt. Set aside. Combine the milk, vanilla extract and lemon juice. Alternately combine the dry ingredients and the milk mixture with the butter mixture. Beat well until smooth. Add the eggs 1 at a time and mix well. Divide the batter between the 2 cake pans. Bake the cake for about 20 minutes until a skewer inserted in the middle comes out clean. Cool before unmolding.
For the Lemon Curd: Whisk together the egg yolks and the sugar. Add the lemon juice. Place the mixture in a heavy bottomed sauce pot. Cook over medium low heat, stirring constantly until thickened. Remove the pot from the heat and stir in the butter. Strain the curd. Stir in the lemon zest. Place the curd in a bowl and cover the surface of the curd with plastic wrap. Refrigerate until cold.
For the Easy Buttercream Frosting: Combine powdered sugar with the other ingredients and beat until smooth.
Assembly: Cut each cake in half, giving you 4 layers. Layer 1 cake piece with 1/3 of the lemon curd. Gently spread about 1/3 cup of the frosting over the curd. Repeat with 2 more layers of cake, curd and frosting. Place the forth cake piece on top and frost the sides and top with the remaining frosting.