In a shallow bowl, whisk together the cooled brewed espresso, 15 ml of powdered sugar, and the coffee liqueur if using. Set aside.

In a chilled mixing bowl, combine the mascarpone cheese, heavy whipping cream, remaining powdered sugar, and vanilla extract. Beat with an electric mixer on medium-high speed until medium-stiff peaks form.

Slice each croissant horizontally across the middle using a serrated knife, keeping the back hinge intact or separating completely.

Generously brush the cut interior surfaces of both top and bottom halves with the prepared espresso syrup using a pastry brush.

Pipe or spoon a thick layer of the mascarpone cream over the bottom half of each croissant. Lightly dust the cream layer with cocoa powder.

Place the top croissant halves over the filling, pipe a small decorative rosette of cream along the top ridge, dust generously with remaining cocoa powder, and top with dark chocolate curls.