In a medium bowl, whisk together gluten free all-purpose flour blend, baking powder, salt, and instant espresso powder. Set aside.
In a large bowl, beat softened butter, 4 oz mascarpone cheese, and granulated sugar with an electric mixer until light and creamy, about 3 minutes. Beat in the egg and vanilla extract until smooth.
Add the flour mixture and mix on low speed until a soft dough forms. Cover and refrigerate for at least 1 hour until the dough is firm enough to scoop.
Preheat your oven to 350 degrees F and line two baking sheets with parchment paper. Scoop rounded tablespoons of dough, roll into balls, and place 2 inches apart on the prepared baking sheets.
Bake for 11 to 13 minutes until the edges are set but the centers are still soft. Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
For the topping, beat 4 oz mascarpone cheese, powdered sugar, and milk together until smooth and thick, like a spreadable frosting. Spread or pipe a generous dollop onto each cooled cookie.
Dust the tops generously with cocoa powder through a fine mesh sieve right before serving. The cocoa powder dusting gives the signature tiramisu look.