Sift together the flour, cocoa powder, salt, and baking powder in a large bowl, and set aside
In a large mixing bowl, cream the butter, brown sugar, and sugar. Add the egg yolk & egg one at a time, then the vanilla, followed by the instant coffee. Mix until combined.
Add the dry ingredients and mix until incorporated.
Wrap the dough with plastic wrap and cool in the refrigerator at least 1 hour.
Divide the dough in half. Knead the dough slightly to help soften and avoid cracks when rolling.
Place one piece between 2 lightly floured sheets of parchment paper and roll into a ¼-inch-thick rectangle. Repeat with the other piece of dough.
Using a 2-inch round cookie cutter, cut the dough into about 64 circles. Any scraps will have to be re-chilled before you can re-roll it.
Place the cookies about 2 inches apart on ungreased baking sheets and chill for 20 minutes. Decorate with a cookie stamp or cookie cutter (if using a cookie-cutter be sure not to cut all the way through the dough.
Preheat the oven to 325f/165c.
Bake the cookies until they are set (about 8-10 minutes) be sure not to let them get too dark or the cookie will become bitter.
Cool completely on cooling racks.
Roll out the fondant, using icing sugar for rolling (not flour) to just over a quarter of an inch thickness and cut into circles using the same cookie-cutter as for the cookies.
Brush one side of the fondant with a little milk and press this onto the bottom of one cookie.
Brush the other side of the fondant with a little water and place the bottom of a second cookie, pressing gently to secure the sandwich. Repeat with the remaining cookies, re-rolling the fondant if needed.
• Be sure to allow the cookies to cool completely before assembling. Placing the fondant too soon will cause the cookies to soften and lose their crunch.• Use powdered sugar instead of flour when dusting the rolling surface to avoid changing the texture if the cookie.