1) Preheat the oven to 350°F. Lightly grease two baking sheets, or line with parchment.
2) To make the cakes: Beat together the butter, sugar, espresso powder, baking powder, baking soda, and salt until smooth.
3) Add the egg, again beating until smooth. Stir in the cocoa.
4) Add the flour alternately with the milk, beating until smooth, and scraping down the sides and bottom of the bowl.
5) Drop the dough by the tablespoonful onto the baking sheets, leaving about 1 1/2" to 2" between the rounds.
6) Bake the cakes for 10 to 12 minutes, until firm to the touch. Cool on the pans for 10 minutes, then transfer to a rack to cool completely.
7) To make the filling: Heat the cream to just below simmering. Pour over the chocolate, let sit for 30 seconds, and stir until smooth, reheating briefly if necessary. Cool to room temperature. Refrigerate for several hours, to set.
8) When ready to fill the cakes, beat the chocolate mixture until it's light and airy.
9) Spread the flat side of half the cakes with the filling. Top with the remaining cakes, rounded side up; press down gently.
Yield: 20 to 22 mini pies.