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  1. Preheat the oven to 325°F. Lightly grease (or line with parchment) two baking sheets.

  2. To make the cookies: Beat the butter and sugar together thoroughly, then beat in the salt, egg, water, and vanilla.

  3. Whisk together the flour, cocoa, and baking powder, and add to the butter mixture, stirring to thoroughly combine. The dough will be stiff, but sticky.

  4. Scoop the dough into balls about the size of a small walnut; they'll weigh about 3/8-ounce each. A very slightly heaped teaspoon cookie scoop works well here. Roll the balls of dough in your hands to make them as smooth as possible; this will ensure your cookies are round.

  5. To make the coating: Mix together the cocoa and granulated sugar.

  6. Dip the flat bottom of a glass in the coating, then flatten each dough ball to a 2" round about 3/16" thick, dipping the glass in the coating each time to keep it from sticking to the dough.

  7. Bake the cookies for 12 minutes. Watch the cookies closely towards the end of the baking time. If you see their edges browning, or smell a whiff of chocolate, take them out of the oven — they're done.

  8. Remove the cookies from the oven, and let them cool right on the pan. While the cookies are cooling, make the filling.

  9. To make the filling: Beat the butter or butter/shortening until it's a bit fluffy. Beat in the salt, espresso powder, and vanilla.

  10. Add the confectioners' sugar and milk, beating until smooth. Scrape the bowl, and beat again, to combine any residue. Adjust the consistency of the filling with additional milk or sugar, if necessary; it should be spreadable.

  11. Turn half the cookies bottom side up, and scoop a ball of filling (about 3/4 ounce) onto each; a teaspoon cookie scoop works well here.

  12. Top the filled bottoms with the remaining cookies, squeezing gently to spread the filling to the edges.

  13. Store cookies for a day or so at room temperature, well wrapped. Freeze for longer storage.