Preheat an oven to 375F.
In a bowl, beat the sugar, egg, butter and vanilla until light and creamy. In another bowl, mix the cocoa, baking soda, salt, and flour together. Mix the dry ingredients and buttermilk alternately into the creamed mixture starting and ending with the dry ingredients. Mix well.
Drop by the heaping tablespoon at least 1 1/2-inches apart onto greased baking sheets and bake until puffed and they spring back when pressed lightly, 7 to 9 minutes. With a spatula, remove from the baking sheet and cool on wire racks.
For the filling, place all of the ingredients in a bowl and mix them together well.
To assemble, take one of the chocolate cakes and spread a heaping tablespoon onto the flat side. Top with another chocolate cake sandwiching the cream in between. Make sure that you divide the cream evenly between the 15 whoopie pies.