Preheat the oven to 350°F and line a baking sheet with parchment paper.
In a large bowl, whisk together the softened butter, brown sugar, and granulated sugar until smooth and well combined.
Add the egg yolks, instant coffee powder, and vanilla extract. Whisk until fully incorporated.
In a separate bowl, whisk together the flour, baking soda, and salt.
Add the dry ingredients to the wet ingredients and fold until a soft cookie dough forms.
Divide the dough into 6 equal portions and place them on the prepared baking sheet, leaving space between each cookie.
Bake for 10–11 minutes, or until the edges are lightly golden and the centers are just set.
Allow the cookies to cool completely before adding the frosting.
In a medium bowl, whisk together the mascarpone cheese, powdered sugar, and vanilla extract until smooth.
Add the heavy cream and whisk gently until the mixture is thick, creamy, and spreadable. Avoid overwhisking.
Once the cookies have completely cooled, spread or pipe the mascarpone cream onto each cookie.
Finish with a light dusting of cocoa powder.