Step 1: Prepare the Cookie Dough

1. Cream Butter and Sugars: In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar together until light and fluffy, about 2-3 minutes. Creaming the butter and sugars properly creates a soft and tender cookie base.
2. Add Egg and Vanilla: Add the egg and vanilla extract to the butter mixture, beating until well combined. Scrape down the sides of the bowl as needed to ensure everything is incorporated.
3. Sift Dry Ingredients: In a separate bowl, whisk together the flour, cornstarch, baking powder, salt, and instant espresso powder. The cornstarch helps create a light and tender texture, while the espresso powder adds a bold coffee flavor.
4. Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix, as this can lead to tougher cookies.

Step 2: Chill the Dough

1. Chill the Dough: Cover the dough with plastic wrap and chill in the refrigerator for at least 30 minutes. Chilling the dough helps the cookies maintain their shape during baking and intensifies the flavors.

Step 3: Bake the Cookies

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper or a silicone baking mat.
2. Portion the Dough: Use a cookie scoop or tablespoon to portion out rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
3. Bake: Bake in the preheated oven for 10-12 minutes, or until the edges are set and the tops look slightly cracked. The centers should still be soft, as the cookies will continue to firm up as they cool.
4. Cool: Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before frosting.

Step 4: Prepare the Mascarpone Frosting

1. Beat Mascarpone and Butter: In a mixing bowl, beat the softened mascarpone cheese and butter together until smooth and creamy.
2. Add Powdered Sugar and Flavoring: Gradually add the powdered sugar, beating until fully incorporated. Add the vanilla extract and espresso or coffee liqueur, if using, and continue to beat until the frosting is smooth and fluffy.

Step 5: Frost and Dust the Cookies

1. Frost the Cookies: Once the cookies are completely cool, spread a layer of mascarpone frosting on top of each cookie using a small offset spatula or piping bag for a decorative touch.
2. Dust with Cocoa Powder: Lightly dust each cookie with unsweetened cocoa powder to give them the classic tiramisu appearance.