• Preheat the oven to 350º and line heavy baking sheets with parchment.
• Mix the espresso powder and hot water together in a small ramekin or bowl, until very smooth - set aside.
• In the bowl of a stand mixer, or in a large bowl, if using a hand mixer, cream together the butter and brown sugar.
• Beat in the egg and vanilla.
• Add the espresso/water mixture, using a rubber scraper to get it all out of the small bowl.
• Whisk the flour, cocoa and salt together in a smaller bowl, and then add in 3 addition to the butter mixture, beating until well combined.
• Follow the directions for your cookie press and press the cookies out directly on the baking sheets. This amount of dough will fill a standard press 4 or 5 times.
• The dough can be refrigerated after mixing if you like [and it does make the flavor a bit richer], but bring the dough back to room temperature before pressing out the cookies.
• Very lightly mist the cookies with water, using a spray bottle - this is optional, but it does help the sugar stick and gives the cookies a pretty sheen.
• Sprinkle with cookie with coarse decorating sugar if desired.
• Bake for 8 to 10 minutes, until the cookies are nicely set.
• Remove to a rack and cool completely before filling.

Make the filling

• Place all ingredients in a medium bowl, and beat with a hand mixer until very smooth and creamy. You want this a little firmer than a frosting for cakes.
• Pipe or spread a small amount of the filling on half of the cookies, and top with the other halves. They are best filled the day they are served, but leftovers will keep in a sealed container in the fridge for up to a week