1) Make the filling first. Place the chocolate chips, corn syrup, and cream into a large microwave-safe bowl, or into a large saucepan.
2) Heat the mixture till it's very hot; the cream will start to form bubbles. Remove from the heat, and stir until smooth.
3) Add the vanilla, then the sifted confectioners' sugar, beating till smooth. Cover the filling and set it aside while you bake and cool the cookies.
4) Preheat the oven to 300°F. Lightly grease (or line with parchment) two baking sheets.
5) Combine the vanilla, baker's ammonia, and salt in a very small bowl. Stir to dissolve the ammonia; the salt won't fully dissolve.
6) Combine the vanilla mixture with the butter, sugar, and espresso powder, and beat till smooth. If you're using baking powder in place of baker's ammonia, beat it in along with the espresso powder.
7) Mix in the cocoa and flour; the mixture will seem quite dry at first.
8) Continue beating until the dough comes together.
9) Break off pieces of dough about the size of a shelled chestnut (about 3/4 ounce, about 1" to 1 1/4"). Roll the pieces into balls. Space them on the prepared baking sheets, leaving 2" between them.
10) Use the bottom of a glass, dipped in cocoa if necessary to prevent sticking, to flatten the balls to about 3/8" thick. If you have it, use the end of the food pusher from a food processor; it's just the right size for this job.
11) Bake the cookies for 20 minutes; they'll feel soft, but set.
12) Remove them from the oven, and immediately cut a hole in the center of each, using an apple corer, a tiny round cutter, or whatever you have on hand to do the job. The hole should be about 3/4" to 1" in diameter.
13) Allow the cookies to cool completely. When they're totally cool, spread with the filling.
14) Use a tablespoon cookie scoop to dollop 1 ounce (about 2 tablespoons) filling on half the cookies. Leave the cookies right on the cookie sheet. Don't worry, once the chocolate sets, you'll be able to remove them using a spatula.
15) Spread the filling nearly to the edge of each cookie.
16) Top the filled cookies with the "bare" cookies, pressing down gently.
17) Allow the chocolate to set completely before moving the cookies.
Yield: about 2 dozen 2 3/8" sandwich cookies.