Butter a 9-inch baking pan, line the base with a round of parchment paper, and flour the pan.
Preheat oven to 350 degrees F. In the bowl of an electric mixer fitted with the paddle attachment, cream together the butter and sugar until light in color and fluffy, about 5 minutes. Add the vanilla extract. Add the eggs 1 at a time. Sift together flour, baking powder, and a pinch of salt 3 times. Add to the egg mixture in 3 increments, alternating with milk, starting and ending with flour. Pour the batter into prepared pan, filling 2/3 full. Bake approximately 35 to 50 minutes; cake is done when the top is golden brown and a tester inserted into the center of the cake comes out clean. Cool cake on cooling rack for 15 minutes. Remove from pan, and continue to cool completely. Using a serrated knife, slice cake in half horizontally. Place the domed layer, dome-side down, on a serving platter. Spread lemon curd on the cake to within 1/2-inch of the edge. Return the top layer, bottom side up. Cake can be refrigerated 3 to 4 days up to this point. Just before serving, place desired stencil over cake. Sift confectioners' sugar or cocoa over cake to fill in exposed areas. Remove stencil, and serve.
In a small saucepan, whisk together yolks, lemon zest, lemon juice, and sugar until well combined. Cook over low heat, stirring constantly with a wooden spoon until it becomes thick enough to coat the back of spoon, about 10 minutes. Remove pan from heat. Stir in butter 1 piece at a time until smooth. Transfer to a small bowl, and cover top of curd directly with plastic wrap. Chill until firm, about 1 hour. Yield: 1 1/2 cups;
(c)2002 Martha Stewart Living Omnimedia, Inc. All Rights Reserved