For the Cookies:
7-1/2 ounces (1-2/3 cups) unbleached all-purpose flour
3/4 ounce (1/4 cup) Dutch-processed cocoa
1/2 teaspoon baking soda
1/4 teaspoon table salt
4 ounces (1/2 cup) unsalted butter, at room temperature
1/2 cup plus 2 tablespoons granulated sugar
6 tablespoons packed light brown sugar
1 large egg
1 tsp. pure vanilla extract
For the Mocha Filling:
1 tablespoon instant espresso
2 ounces (4 tablespoons) unsalted butter, at room temperature
2 ounces (4 tablespoons) cream cheese
6 ounces (1 1/2 cups) confectioners' sugar, sifted
1 teaspoon pure vanilla extract
3/4 ounce (1/4 cup) Dutch-processed cocoa powder, sifted
In a medium bowl, whisk the flour, cocoa, baking soda, and salt. With a hand mixer or a stand mixer fitted with the paddle attachment, cream the butter and sugars on medium speed until light and fluffy, about 2 minutes. Add the egg and vanilla and continue beating until blended and smooth, about 30 seconds. Reduce the speed to low and slowly add the dry ingredients, mixing until the dough is just combined. Divide the dough in half. Wrap one half of the dough in plastic and refrigerate.
Roll the other half of the dough between two sheets of parchment to an even 1/8-inch thickness. Slide the dough onto a cookie sheet and freeze until cold and firm, about 30 minutes. Repeat with the remaining dough.
Position a rack in the center of the oven and heat the oven to 350°F. Line two cookie sheets with parchment.
Using 1-1/2-inch round cookie cutters, cut out the dough and arrange the rounds 1 inch apart on the prepared sheets. If the dough gets too soft, return it to the freezer for a few minutes. Carefully press the scraps together, reroll, and cut. Repeat with the other half of the dough, and then gather all the scraps together, reroll, and cut one more time.
Bake in batches, two sheets at a time, until the tops look dry, about 6 minutes. Let the cookies cool on their pans for a minute and then let them cool completely on racks.
Make the filling:
In a small bowl, dissolve the espresso in 2 tablespoons hot water. Let cool slightly, 5 minutes.
With the mixer, cream the butter and cream cheese on medium speed until light and smooth, about 1 minute. Reduce the speed to low and slowly add half of the sugar, mixing until just combined. Add the coffee mixture and vanilla, and mix until just incorporated. Gradually mix in the remaining sugar and the cocoa. Increase the speed to medium and beat until the filling is light and fluffy, about 1 minute more.
To assemble, transfer the cooled cookies to a work surface, flipping half of them over. With an offset spatula or butter knife, spread a thin layer of the filling onto each turned-over cookie. Set another cookie on top of each filled cookie, pressing gently to spread the filling. The cookies will keep at room temperature for up to 3 days or in the freezer for up to 1 month.
Make Ahead Tips
The filling can be made up to 3 days ahead and refrigerated. For the best texture, assemble the sandwiches as close to serving as possible.