dough:
115 g butter
115 g sugar
1 egg yolk
1 tb whipping cream
120 g flour
60 g cocoa powder
1 TL coarse sea salt
ganache:
70 g whipping cream
60 g chocolate, 75%, small pieces
1 ts (lean) ground espresso beans
Whisk butter and sugar until creamy. Add egg yolk and whipping cream. Beat in flour, cocoa and salt until just combined. Using your hands, quickly knead to a smooth dough. Shape a roll and refrigerate for about 30 minutes.
Divide the dough into 20 (or more) pieces. Shape little balls. Cut each ball into halves and place on a parchment-lined baking sheet. With the handle of a wooden spoon, press in the center of each cookie to create an indentation. Refrigerate for about 10 minutes; then bake in preheated oven at 175 deg C for about 10 minutes or until the cookies are just set. Using the parchment paper, transfer cookies onto a rack. Let cool.
For the ganache, combine whipping cream and ground espresso beans. Bring to a boil, remove from heat and pour through a fine sieve over the chocolate. Let stand for about 2-3 minutes, then whisk until smooth. Fill the ganache into a pastry bag and pipe into the center of each cookie.
Because of the ganache, eat the cookies within 3 days. But it won't take that long. :-)
(Recipe modified; original sources: Il pranzo di Babette / Martha Stewart)