Add the egg and egg yolk, and mix again until the mixture is smooth and fully blended.
Add the flour and baking soda, and mix on low speed until the dough is almost combined. Avoid overmixing.
Use a large cookie scoop—or about 2 tablespoons of dough—to form the cookies. Place the dough balls on a parchment-lined baking sheet.
Bake in a preheated 180°C (350°F) oven for 10–12 minutes, or until the edges are lightly golden. The centers should look soft—this keeps the cookies chewy.
Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.
Make the mascarpone cream
In a large bowl, add the heavy cream, espresso powder, vanilla extract, and powdered sugar. Beat with an electric hand mixer until stiff peaks form. (Be careful not to overmix, or the cream may turn into butter.)
Add the room-temperature mascarpone cheese and gently fold it into the whipped cream with a spatula until smooth and creamy.
Assembly
Transfer the mascarpone cream into a piping bag fitted with a round tip and pipe a generous swirl onto each cooled cookie. Dust with cocoa powder and serve!
Notes
Use room-temperature butter and mascarpone. Softened butter creams smoothly, and room-temp mascarpone blends without lumps.
Measure the flour correctly. Too much flour makes cookies dry and cakey. For best accuracy, use a kitchen scale. If using cups, spoon, and level, never scoop.
Don’t overmix the whipped cream. Stop mixing as soon as stiff peaks form, or the cream can turn grainy or buttery.
Use good-quality espresso powder. The flavor relies heavily on it, so choose a rich, strong espresso powder for the best taste.
Mix the dry ingredients gently. Once the flour is added, mix only until just combined to keep the cookies soft and chewy.
Bake until just set. Remove the cookies when the edges are lightly golden, and the centers still look soft. They’ll firm up as they cool.
Cool completely before frosting. Warm cookies will melt the mascarpone cream, so let them cool fully on a wire rack.
Use a cookie scoop. It keeps all the cookies the same size, which helps them bake evenly.
Fold the mascarpone gently. Mix it into the whipped cream with a spatula so the frosting stays light and silky.
Dust with Dutch-processed cocoa powder. It has a deep, smooth flavor and gives the cookies that classic tiramisu finish.