Start by melting the unsalted butter in a mixing bowl. Combine it with brown and granulated sugar, beating until creamy and light.

In a separate bowl, sift together all-purpose flour, cocoa powder, baking powder, baking soda, and salt. Gradually fold this into the wet ingredients along with the Oreo crumbs.

Cover the dough and let it rest for about 10 minutes while preheating the oven to 350°F (175°C).

Portion the dough onto lined baking sheets, spacing them 2 inches apart. Aim to form about 12-14 cookies.

Bake for 11 minutes, then press the centers down for mousse wells. Return to the oven for 2 more minutes.

Cool the cookies on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.

Whip heavy cream, powdered sugar, cocoa powder, and vanilla until thick. Fold in the pudding mix combined with milk to make the mousse.

Once cooled, fill the cookie wells with chocolate mousse and top with whipped cream. Chill for at least 30 minutes before serving.

Consider adding chocolate curls or a dusting of powdered sugar for garnish before serving.