Start by melting the butter in a medium bowl. Using a hand mixer, beat the melted butter with brown and granulated sugars until creamy, about 2-3 minutes. Next, add the room temperature eggs and vanilla extract, mixing well until fully incorporated.

In a separate bowl, sift together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt. Gradually fold the dry mixture into the wet ingredients using a spatula, being careful to avoid overmixing.

Once the dough is combined, allow it to rest for about 10 minutes. While the dough rests, preheat your oven to 350°F (175°C), and prepare two baking sheets by lining them with parchment paper.

Using a cookie scoop or spoon, portion the dough onto the prepared baking sheets, spacing them about two inches apart. Bake for 11 minutes, then press centers of the cookies down gently with a measuring cup.

Bake for an additional 2 minutes until they are soft yet set, and the edges begin to crisp.

Once baked, remove the cookies from the oven and allow them to cool slightly on the baking sheet for about 5 minutes. Then, transfer them to wire racks to cool completely.

To prepare the mousse, chill your mixing bowl and whisk for about 10 minutes. Once chilled, whip the heavy cream, powdered sugar, cocoa, and vanilla extract together until stiff peaks form, about 4-5 minutes.

Gently fold in the instant chocolate pudding that has been mixed with milk until fully combined, creating a rich and creamy filling ready for your cookies.

With the cookies completely cooled, use a spoon to fill each well in the center of the cookies with generous amounts of the chocolate mousse. Top each filled cookie with a dollop of whipped cream.

To allow the mousse to set properly, place the assembled cookies in the refrigerator for at least 30 minutes.