Preheat oven to 350 degrees and line one large cookie sheet with parchment paper. In a large bowl, mix flour, baking powder, and salt. Set aside.
In the bowl of your stand mixer (or a large bowl with a hand mixer) beat butter and both sugars until pale and fluffy, a full three minutes. Beat in instant coffee granules. Then, beat in eggs, one at a time, being sure to incorporate each one completely before adding another. Beat in vanilla extract (if using) and scrape down the sides of the bowl as needed.
On the slowest speed possible, mix flour mixture into butter mixture until dough comes together.
Divide dough in half and shape into two logs, approximately 9 inches long. Pat each log down so that it is only about 1 1/2 inch high. Place logs on prepared cookie sheet with at least 2 inches of space between for spreading.
Bake at 350 for 35 minutes, or until the top just begins to crack and brown. Let cool for 10 minutes before removing to a cutting board. Using a large serrated knife, cut logs on the diagonal into slices roughly 1 inch thick.
Place sliced cookies back onto the baking sheet and bake for an additional 12-15 minutes or until completely dry in the center. Let cool on wire rack.
When cool, drizzle with 1/2 cup melted chocolate and top with sprinkles!