In a small bowl whisk together flour, espresso powder, and salt. Set aside.
In a large bowl beat together softened butter and granulated sugar with an electric mixer on medium speed until soft and creamy. Beat in egg yolk, then beat in rum extract and vanilla extract until well combined. Beat in mascarpone cheese until just combined.
Beat in flour mixture to butter/sugar mixture until just combined. Scoop dough onto a large piece of plastic wrap, flatten into a large rectangle, and cover with wrap. Refrigerate at least 1 hour or overnight.
Preheat oven to 350 degrees F. Line baking sheets with parchment paper or silicone mats.
Roll out chilled dough with a rolling pin to about 1/4-inch thickness or so. Use a 1 1/2-inch square cookie cutter to cut out dough. Place cutout dough onto prepared baking sheets. The dough does not spread much while baking, so you can place fairly close together.
Sprinkle a little unsweetened cocoa powder on the top of each cookie and spread with your finger to cover tops of each cookie.
Bake cookies about 10-12 minutes or until golden brown underneath. Cool slightly then place on wire cooking racks to cool completely.
SNAPPY TIPS: You can roll the cookie dough thinner or thicker and also cut into any desired shape but you may need to adjust the baking time. Keep the dough as chilled as possible while cutting. You can cut and bake in batches if you'd like. SNAPPY SUBSTITUTIONS: Instead of instant espresso, you could try instant coffee, but it may change the taste a bit of the cookies. Instead of mascarpone cheese, you could try cream cheese.