Sift flour and cocoa in a medium bowl. Cream butter and sugar in mixer. Add vanilla and salt and beat until smooth. Add flour mixture and mix on low until soft dough forms. Refrigerate for an hour. Preheat oven to 350°F. Roll into small 1 teaspoon size balls and place 2 inches apart on baking sheets. Make an indent in each ball using the end of a wooden spoon. Bake until top of cookie looks dry, approx. 8 minutes. Re-indent the cookies before they cool.

To make the filling: Place chocolate and butter in a large microwave safe bowl. Heat at 30 second intervals, stirring in between. Allow to cool slightly. Stir in the peppermint extract at this stage if you ar using it. When cookies are cool fill with chocolate filling. Add sprinkles before the chocolate sets.