Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.

In a large bowl, beat butter, brown sugar, and granulated sugar until fluffy. Mix in egg, vanilla, and espresso powder.

In another bowl, whisk together flour, baking soda, baking powder, and salt. Gradually add dry ingredients to the wet ingredients, mixing until just combined. Fold in white chocolate chips if using.

Scoop dough into 1.5 tbsp-sized balls and place on the baking sheet. Bake for 10-12 minutes, until edges are golden. Let cool completely.

For the filling, beat together mascarpone, powdered sugar, and vanilla until smooth.

Spread or pipe mascarpone filling onto the bottom of one cookie. Top with another cookie to create a sandwich.

Mix cocoa powder and espresso powder, then dust over cookies. Serve immediately or chill for a firmer texture.