First, preheat oven to 350° F. Line baking sheet with parchment paper. In a medium bowl, whisk together the flour, cocoa, baking soda, espresso powder and salt. Set aside.
In the bowl of a stand mixer (or using a hand mixer), cream together the butter and brown sugar, about 1 minute. Add the egg and vanilla extract and beat until light and creamy, about 2 minutes.
Add dry ingredients to butter mixture in three parts, alternating with buttermilk (so add one third flour mixture, half cup buttermilk, another third flour mixture, half cup buttermilk and finish with remaining flour mixture). Combine well after each addition.
Pipe batter into circles two inches in diameter on the prepared baking sheet (each about three inches apart as they will spread slightly). Bake until tops are puffed and cakes spring back when touched, 10 to 12 minutes. Remove from oven and make the filling!