Add the butter and semi-sweet chocolate chips to a medium saucepan and melt over medium heat, stirring frequently.

Remove from heat and allow to rest until cool for 10 – 15 minutes.

In a mixing bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt and set aside.

In the bowl of a mixer, whip together the light-brown sugar, granulated sugar, eggs, and vanilla on medium-high speed until pale and fluffy, about 1 minute.

Using a rubber spatula stir in the melted chocolate mixture. Working in 3 separate batches, add 1/3 of the flour mixture and fold just until combined.

Chill dough for about 45 minutes. Preheating the oven to 325 degrees during the last 10 minutes of chilling.

Scoop a tablespoon of the dough out and shape it into balls.

Place onto Silpat or parchment paper lined baking sheets, spacing them 2 inches apart.

Bake in preheated oven 12 – 13 minutes, then remove from oven and allow to cool on baking sheet 5 minutes before transferring to a wire rack to cool completely.

In a mixer bowl, cream together the butter and sugars until it's smooth.

Mix in the milk and vanilla until combined. Mix in the flour until just combined.

Then stir in the chocolate chips.

Pull aside half of the cookies and lay them flat on a baking sheet.

Scoop the filling onto a cookie, shape to fit the cookie. Repeat for all of the cookies on the baking sheet.

Take the other baked cookies and place one on top of each cookie sandwich.