(adapted from Alice Medrich's Chocolate and The Art of Low Fat Desserts)
2 cups cake flour
1 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1 teaspoon instant espresso or coffee powder
1/3 cup dutch-process cocoa powder
1 1/3 cups sugar
1 whole egg
1 egg white
6 Tablespoons unsalted butter
1 cup nonfat yogurt
2 teaspoons vanilla extract
Have all ingredients at room temperature. Preheat the oven to 350F. Prepare the pan. Combine the flour with baking powder, baking soda, and salt. Sift together. Set aside.
In a small bowl, combine espresso powder, cocoa, and 1/3 cup of the sugar with 1/4 cup water. Whisk until smooth. Set aside.
In another small bowl, whisk whole egg with egg white. Set aside.
Cut the butter into pieces and place in an electric mixer bowl. Beat for about 1 minute to soften. Gradually add remaining 1 cup of sugar and beat at high speed for about 3 minutes. Dribble eggs in slowly, about 1 tablespoon at a time, beating constantly for about 2 minutes. On low speed, beat in a third of the flour mixture. On medium-high speed, beat in half of the yogurt. On low speed, beat in half of the remaining flour. On high speed, beat in the rest of the yogurt and the vanilla. On low speed, beat in the remaining flour.
Measure out 1-1/2 cups of yogurt batter and mix into the cocoa mixture; set aside.
Use a large spoon to fill the bottom of the pan with about three quarters of the white batter placed in dollops. Cover the white batter with dollops of chocolate batter. Top the chocolate batter with small dollops of white batter spread so that the chocolate batter shows through. Use a table knife to marble the batters together with a circular or zigzag motion; be careful not to blend them too much.
Bake for 40 to 50 minutes, or until cake begins to pull away from the sides of the pan and a toothpick inserted into the center of the cake in several places comes out clean. Cool for 10 to 15 minutes on a rack. Unmold the cake. Cool completely before serving or storing.