With its luscious, fanciful swirls, marble cake may seem the work of a kitchen magician. But the truth is, this recipe -- which owes its two-tone look to gentle mixing techniques -- is easy enough even for kids to whip up. Here's the perfect recipe to make and enjoy on a quiet afternoon.
Lightly butter a 9- by 13- inch cake pan and dust it with flour, knocking out the excess. Heat your oven to 350º.
Combine the dry ingredients by sifting the flour, baking powder, baking soda, and salt into a large bowl. Set the mixture aside.
Melt the chocolate in the microwave or on the stovetop according to the package directions. (Chocolate chips retain some of their shape when melted, so be careful not to overheat or burn them.) Stir the chips until they are smooth, then cool them to lukewarm, about 9 minutes.
Using an electric mixer, cream the butter and sugar on medium-high speed, adding the sugar gradually, until the mixture is light and fluffy. Beat in the eggs one at a time, first breaking them into a separate dish to ensure that no eggshells get in the batter. Add the vanilla extract and blend briefly.
Add about a third of the dry ingredients to the creamed mixture, beating on low speed until the batter is evenly blended. Next, beat in half of the buttermilk until it is evenly blended. Continue in this manner, adding another third of the dry ingredients, the rest of the buttermilk, then the rest of the dry ingredients, beating well after each addition.
Transfer the chocolate to a medium mixing bowl, then add 2 cups of the batter. Thoroughly fold in the chocolate.
TIP: Folding is a technique for gently incorporating a heavier batch of ingredients into a lighter mixture so the result stays fluffy. In a mixing bowl, pour the lighter batch (here, the plain batter) on top of the heavier ingredients (the chocolate). Slice down through both mixtures with a rubber spatula, draw it across the bottom of the mixing bowl and then up, gently turning over the batter. Rotate the bowl a quarter turn and repeat until all the ingredients are thoroughly mixed.
Transfer a little more than half of the remaining plain batter to the prepared pan, spreading it evenly with the back of a spoon. Using another large spoon, make 3 wide, lengthwise rows of chocolate batter on top, as shown. Switch back to the plain batter and use what's left to fill between the chocolate rows.
To create a swirled pattern in the batter, draw the top edge of a table knife across the rows, working from side to side. For best results, be sure not to remove the knife from the batter as you work.
Bake the cake on the center rack until a toothpick inserted in the center comes out clean, about 40 to 45 minutes. Set the pan on a wire rack and let the cake cool thoroughly before applying the glaze.
To make the glaze, melt the chocolate chips in the microwave or on the stovetop according to the package instructions. Stir the chocolate until it is smooth, then whisk in the butter, confectioners' sugar, water, and vanilla extract until the glaze is fluid.
Using a large spoon, drizzle the mixture over the cake right away. The icing will firm up somewhat as it cools, but if it stiffens before you finish, you can reheat it to make it more liquid. Serves 12.