Prepare the Cookie Dough: In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. In a separate large bowl, beat the softened butter, granulated sugar, and brown sugar until light and fluffy. Add the egg and vanilla extract and mix until smooth. Stir the instant espresso powder into the milk, then add the mixture to the wet ingredients. Gradually add the dry ingredients and mix just until a soft dough forms. Avoid overmixing.
Chill the Dough: Cover the cookie dough and refrigerate for about 30 minutes. Chilling helps firm the butter and prevents the cookies from spreading too much while baking.
Shape the Cookies: Preheat the oven to 350°F (175°C). Line baking sheets with parchment paper. Scoop the chilled dough into evenly sized portions and gently roll each portion into a ball. Place the dough balls on the prepared baking sheets, leaving enough space between them for spreading.
Bake the Cookies: Bake for 9–11 minutes, or until the edges are set and the centers still look slightly soft. Be careful not to overbake the cookies. They will continue to set as they cool, which helps keep the centers soft and tender.
Cool Completely: Let the cookies cool on the baking sheet for several minutes before transferring them to a wire cooling rack. Allow the cookies to cool completely before adding the mascarpone topping.
Make the Mascarpone Topping: In a clean bowl, beat the mascarpone cheese, powdered sugar, and vanilla extract until smooth. Gradually add the heavy whipping cream and continue beating until the mixture is creamy, fluffy, and easy to pipe. Avoid overmixing.
Top the Cookies: Transfer the mascarpone topping to a piping bag fitted with your preferred tip. Pipe a generous swirl of mascarpone topping onto each completely cooled cookie. You can also spread the topping with a small spatula for a simple homemade finish.
Finish With Cocoa: Using a fine-mesh sieve, lightly dust unsweetened cocoa powder over the mascarpone topping. Refrigerate the finished cookies until ready to serve. For the best flavor and texture, let them sit at room temperature for a few minutes before enjoying.