Heat oven to 350 degrees F.
Butter a 9-inch round cake pan; line with a circle of parchment. Dust the bottom and sides of the pan with cocoa; tap out any excess. Sift cocoa; whisk with 1/2 cup boiling water. Set aside to cool. In a large bowl, sift together the flour, baking soda, and salt. Set aside. In the bowl of an electric mixer fitted with the paddle attachment, cream butter on medium speed until light and fluffy, 3 to 4 minutes. Gradually add granulated sugar; beat on high until light and fluffy, about 5 minutes, scraping down the sides twice. Add the vanilla and eggs 1 at time, beating well after each addition, scraping down the sides when necessary. Whisk together the cooled cocoa and the milk until well combined. Slowly add to the butter mixture; beat until light and fluffy, about 2 minutes. Add the dry ingredients, and mix on low until just combined. Pour batter into the prepared pan. Transfer to oven, and bake 20 minutes. Rotate pan, and bake 15 to 20 minutes more, or until a cake tester inserted in the middle of the cake comes out clean. Transfer pan to a rack to cool. Allow cake to cool in pan for 15 minutes. Carefully run a knife around the edge of the cake to loosen it from the pan. Invert the cake onto a cooling rack. (The bottom of the cake will now serve as the top of the cake.) Carefully remove the parchment circle, trying to maintain as smooth a surface as possible. Allow the cake to cool completely. If the cake is not sitting flat, turn it over, and slice off the dome using a serrated knife. Using a serrated knife, slice cake in half horizontally. Place the domed layer, dome-side down, on a serving platter. Spread raspberry jam 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.
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