Prepare the cookie dough by mixing butter, brown sugar, and granulated sugar until smooth. Add the egg, vanilla extract, and dissolved coffee, then mix until combined.
In a separate bowl, combine flour, cocoa powder, salt, baking powder, and baking soda. Gradually add the dry ingredients to the wet mixture until a soft dough forms. Cover and chill for 1–2 hours.
Prepare the mascarpone filling by whisking egg yolks and sugar over a water bath until creamy. Remove from heat and continue mixing until slightly cooled.
Mix mascarpone cheese, vanilla extract, strawberry powder, and the egg yolk mixture until smooth. Whip the cream separately, then fold it into the mascarpone mixture until creamy.
Shape the chilled cookie dough into portions and place them on a lined baking sheet. Press slightly to create the desired cookie shape.
Bake in a preheated oven at 180°C (356°F) for 12–14 minutes. Immediately after baking, create a small center indentation and allow the cookies to cool completely.
Fill the cooled cookies with the strawberry mascarpone cream and smooth the top.
Decorate with strawberry powder and fresh strawberry slices before serving.