1) Preheat the oven to 325°F. Lightly grease (or line with parchment) two baking sheets, or more if you have them.
2) To make the cookies: In a medium-sized mixing bowl, beat together the sugar, butter, salt, and espresso powder. That's right; there's no leavening in this recipe, so don't worry that something's been left out.
3) Beat in the egg, water, and vanilla, then the flour and cocoa. The dough will be very stiff.
4) Roll the dough into balls about the size of a chestnut (about 2 level teaspoons). A teaspoon cookie scoop works fabulously here, as well as for the filling. If you don't have one, consider a purchase; you won't regret it. Place the dough balls on the prepared baking sheets, leaving about 1 1/2" to 2" between them.
5) Use the flat bottom of a glass, dipped in cocoa as necessary to prevent sticking, to flatten the cookies to about 1/8" to 3/16" thick. The end of a food processor's pusher tool works well here, too. Take a ruler and measure the cookies' thickness; you want to get pretty close to this measurement, for the best-textured cookies.
6) Bake the cookies for 18 to 20 minutes. It's important to bake them just the right amount of time; too little, and they won't be crisp; too much, and they'll scorch. Watch them closely at the end of the baking time, and if you start to smell scorching chocolate before the time is up, take them out. When they're done, remove the cookies from the oven, and allow them to cool completely, on a rack or on the pan.
7) While the cookies are cooling, make the filling.
8) Beat together the sugar, shortening, and vanilla. It'll seem very dry at first, but will eventually begin to clump together.
9) Add the water, beating till smooth and spreadable. The filling should be stiff, but not so stiff that you can't flatten it when you sandwich it between the cookies.
10) Place one level tablespoon filling in the center of one cookie; again, a teaspoon cookie scoop, slightly heaped, is perfect for this task. Place another cookie atop the filling, and squeeze to distribute the filling evenly. Repeat with the remaining cookies. Store in an airtight container.