1. Prepare the cake. Preheat oven to 350 degrees F. Butter and flour 2 (9-inch) round cake pans.
2. In a medium bowl, whisk together flour, cocoa, baking soda, baking powder, andsalt. Set aside.
3. In a large bowl, use an electric mixer on medium-high speed to beat 1 cup butter with the sugar until fluffy, about 4 minutes. Scrape down the sides, then add eggs, one at a time, beating until each is incorporated. Scrape downthe sides, then add vanilla and beat for 2 minutes longer to build structure.
4. Add half the dry ingredients and mix just until combined. Add half the yogurt, mix, then repeat with remaining dry ingredients and 1 1/4 cups yogurt, mixing just until incorporated. The batter will be very thick. Divide evenly between the prepared pans, smoothing the tops. Bake for 25 to 30 minutes, until a toothpick inserted in the center comes out clean. Be careful not to overbake. Let cakes cool in pans for 10 minutes, then run a knife around edges of pans to loosen and invert layers onto racks. Cool completely. (Cake layers may bemade 1 day ahead and stored, well wrapped in plastic wrap, at room temperature, or frozen in freezer storage bags for several weeks.)
5. Prepare the peanut butter filling. Place butter and white chocolate in the top of a double boiler set over just steaming water. Melt over medium-low heat until 3?4 of the butter and chocolate are melted. Remove from heat and stir until melted completely and smooth. Stir in peanut butter, then add confectioners' sugar and salt, mixing until very smooth. Set aside to cool andthicken slightly.
6. Prepare the ganache. Heat cream and corn syrup in the top of a double boiler over steaming water for 2 minutes, until hot. Remove from heat and add chopped chocolate, stirring until melted. Add yogurt and stir to incorporate. Allow to cool slightly, so the ganache is thick enough to cling to the top of the cake, but still pourable.
7. Assemble the cake. Place 1 layer on a serving plate, flat side up. Spread peanut butter filling over the cake, then top with second layer, flat side down. Spoon or pour ganache over the top layer, spreading over the surface, allowing a little to dribble over the edges. Let the ganache set before serving, at least 30 minutes. Serve at room temperature.
Source: Fresh Magazine March, April 2010