In a large mixing bowl, beat 1 cup of cooled melted butter with 1/2 cup brown sugar and 1/2 cup granulated sugar until creamy and well combined, about 2 minutes. Add in 2 large eggs and 1 tablespoon of vanilla extract and mix until incorporated.

Sift together 1 3/4 cups all-purpose flour, 1/2 cup cocoa powder, 1/2 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt in a separate bowl. Gently fold the dry mixture into the wet ingredients along with 1 cup of Oreo crumbs.

Let the cookie dough rest for 10 minutes at room temperature. Preheat oven to 350°F (175°C) and line baking sheets with parchment paper. Use a cookie scoop to drop dough balls, and bake for 11 minutes.

Press the centers gently with a measuring cup to create a well for the filling. Bake for an additional 2 minutes until edges are firm.

Transfer cookies to wire racks and allow them to cool completely, ideally for 15-20 minutes.

Chill your mixing bowl and whisk in the freezer. Whip 1 cup heavy cream with 1/2 cup powdered sugar and 1/4 cup cocoa powder until thick. Fold in instant chocolate pudding prepared as per instructions.

Scoop the chocolate mousse filling into the wells created earlier, generously filling each one.

Optionally sprinkle cookies with chocolate curls or powdered sugar. Refrigerate for at least 30 minutes to set the mousse.