Stir together the first three ingredients until dissolved. Set aside.
Beat the butter, sugar and egg till fluffly, add the coffee mixture and slowly blend in the flour. Divide dough in half, wrap in plastic wrap and refrigerate for one hour. Divide each portion into 24 balls and press into ungreased mini-tart pans. Bake in a pre-heated 375* oven for 8-10 minutes. Remove from oven and tamp the centers of the tarts down, allow to cool in the pans for 5-10 minutes.
In a medium bowl beat the remaining butter, powdered sugar, milk, vanilla and coffee liquor till smooth and fluffy.
Pipe or spoon the filling into the cooled shells and dust with the cocoa powder.