Step 1

Lightly grease (or line with parchment) two baking sheets, or more if you have them; you'll make 3 to 4 baking sheets' worth of cookies., To make the cookies: In a medium-sized bowl, beat together the sugar, butter, salt, baking powder, and espresso powder., Beat in the egg and vanilla, then the flour and cocoa. Cover the dough, and chill for 30 minutes. While the dough is chilling, preheat the oven to 350°F., Roll the dough about 1/8" thick; use cocoa instead of flour to dust your rolling board and the dough., Cut into 2 1/2"-round cookies. A biscuit cutter is handy for this., Bake the cookies for 20 minutes. Watch them closely at the end of the baking time, and if you start to smell chocolate before 20 minutes has gone by, take them out., When the cookies are done baking, remove them from the oven and allow them to cool completely.

Step 2

, To fill the cookies: Use a small offset spatula or a knife to spread about 2 teaspoons of filling onto the bottom of one cookie. Or for a cleaner presentation, roll about 2 teaspoons of filling in the palms of your hands to form a ball, and place it on the bottom of a cookie.

Step 3

, Finish the sandwich cookie by placing another cookie, bottom-side down, on top of the filling. Press gently, until the filling almost reaches the edge of the cookies.

Step 4

, Repeat with remaining cookies and filling, until all are filled.

Step 5

, Storage information: Store well wrapped, at room temperature for several days; freeze for longer storage.