Preheat the oven to 350F. Line two cookie sheets with parchment paper.
In a medium bowl, whisk together the flour, cocoa powder, baking powder, salt, and espresso powder.
1 ⅓ cups all purpose flour, ½ cup Dutch process cocoa powder, ½ teaspoon baking powder, ½ teaspoon fine sea salt, ½ teaspoon espresso powder, or instant coffee, optional
In the bowl of a stand mixer, cream the butter, brown sugar and granulated sugar, beating for about 3 minutes.
¾ cup unsalted butter, room temperature, ⅔ cup brown sugar, ⅓ cup granulated sugar
Beat in the egg yolks and vanilla.
2 large egg yolks, 2 teaspoon pure vanilla extract
Scrape down the sides and bottom of the bowl with a spatula.
Beat in the flour/cocoa mixture, mixing just until combined.
Portion out the dough using a 1 tablespoon ice cream scoop or measuring spoon.
Roll the portioned dough into balls.
Use the back of a ½ teaspoon measuring spoon, or your thumb to make indentations in the center of the dough balls.
Chill the dough balls for 30 minutes.
Bake for 9-11 minutes, until the cookies look just set.
If the cookies poof up a bit in the center, use a 1 teaspoon measuring spoon to gently make the thumbprint shape again.
Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.