Cookies:

Sift the flour and cocoa together into a medium bowl. With a hand mixer, cream the butter and sugar together on medium ntil light and fluffy, about 2 mins. Add vanilla and salt. Beat until blended and smooth. Add the flour-cocoa mixture and mix on low until dough forms, about 1 minute. Chill the dough in the fridge until firm enough to roll into balls, about 1 hour. Position rack in center of oven and preheat to 350F. Line 2 cookie sheets with parchment paper.

Using your palms, roll a gollfball-sized ball out, arranging them 2 inches apart on the sheets. With a lightly floured thumb or index finger, push straight down into the middle of each dough ball almost to the cookie sheet to make a deep well. Bake one sheet at a time until the tops of the cookies look dry, about 8-9 minutes. Gentline redefine the indentations with the end of a whisk or wooden spoon. Let the cookies cool on the sheet for 5 minutes and then let them cool completely on the racks.

For the frosting, put the chocolate and butter in a heatproof bowl set in a wide skillet of just barely simmering water. Stir with a heatproof spatula until almost melted, 2-4 minutes. Remove the bowl and stir until melted, about 30 seconds more. Stir in the mint extract. Let the filling cool, stirring occasionally until slightly thickened and a bit warmer than room temperature, 30 to 40 minutes. Pipe the filling into the cookies. Cool completely before serving or storing.

Optionally top with sprinkles, powdered sugar or candied mint.