Sift the flour, cocoa powder, salt, and baking powder into a large bowl, and set it aside.
Cream the butter and sugars in a large mixing bowl. Add the egg yolk & egg, followed by the vanilla, then the instant coffee. Mix until combined, and be sure to scrape and mix in anything that sticks to the side of the bowl.
Mix in the dry ingredients until they are incorporated.
Wrap the dough with plastic wrap and cool in the refrigerator for at least 1 hour.
Divide the dough in half. To soften and prevent cracks when rolling, knead the dough slightly.
Roll one half between two sheets of lightly floured parchment paper into a 1⁄4-inch-thick rectangle. Repeat with the other piece of dough.
Cut the dough into 64 circles with a 2-inch round cookie cutter. If there are any scraps, you will need to re-chill them before rolling them again.
Place the cookies two 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 through the dough).
Preheat the oven to 325°F/165°C.
The cookies should be baked until they are set (about 8 to 10 minutes). Do not let them get too dark, or they will become bitter.
Allow to cool completely on cooling racks.
Using the same cookie cutter as the cookies, roll out the fondant using powdered sugar instead of flour to just over a quarter-inch thickness.
Brush one side of the fondant with a bit of water and press this onto the bottom of one cookie.
Brush the other side of the fondant 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.
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Notes
• 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.