Step 1Heat the oven to 350 degrees. Line 22 cups of 2 (12-cup) muffin pans with paper liners.
Step 2Beat together the cream cheese, egg and 1/3 cup sugar until smooth. Stir in the chocolate chips.
Step 3Sift together the flour, remaining sugar, cocoa, soda and salt. In another bowl, whisk together the water, oil, vinegar and vanilla; gradually add to the dry ingredients until blended.
Step 4Fill the cups half full. Top each with 1 tablespoon of filling. Sprinkle with nuts. Bake until the tops spring back, 25 minutes. Fill a pastry bag fitted with a small round tip with ganache. Pipe over the top of the cooled cakes.
Step 1Melt the chocolate chips in the top of a double boiler over simmering water. Blend in the sour cream until smooth. Let cool.