Preheat oven to 375°F. With an electric mixer, beat the sugar, egg, butter, and vanilla until light and creamy. In another bowl, mix the cocoa, baking soda, salt and flour; then blend with buttermilk into the creamed mixture.
Drop batter by the heaping tablespoon onto greased baking sheets and bake 7 to 9 minutes. Remove from the baking sheets and cool on wire racks. Makes 30 mini cakes.
To make the filling, mix together all ingredients well.
To assemble, sandwich a heaping tablespoon of cream between two cakes, dividing cream evenly among the 15 Whoopie Pies.