For the Cookies:
Preheat the oven to 375˚ F. Line baking sheets with parchment paper and set aside.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, salt and espresso powder and set aside. Place the chopped chocolate in a heatproof bowl and melt over a pot of simmering water (or in short intervals in the microwave) until smooth. Set aside and let cool until just barely warm.
In the bowl of an electric mixer, combine the butter and sugar and beat on medium-high speed until light and fluffy, about 2 minutes. Beat in the egg and vanilla. Beat in the melted chocolate until smooth. With the mixer on low speed, add in half of the dry ingredients, mixing just until incorporated. Mix in the buttermilk and mix then mix in the remaining dry ingredients.
Drop the dough in spoonfuls onto the prepared baking sheets, about 1½ tablespoons per cookie. Bake 8-10 minutes, just until set, rotating the pans halfway through baking. Let cool on the baking sheet about 2 minutes then transfer to a wire rack to cool.
For the frosting:
Place the chocolate chips in a heatproof bowl. Melt over a pan of simmering water or in the microwave in short intervals, stirring until smooth.
Stir in the sour cream. Whisk in the confectioners’ sugar until the mixture is smooth and glossy. If necessary adjust the consistency with additional confectioners’ sugar to thicken or sour cream to thin it out.
Frost cookies as desired and decorate with sprinkles.