Preheat the oven 350ºF. Grease or line with parchment paper a large baking sheet.
In a bowl, mix together all-purpose flour, cocoa powder, baking powder, and salt. Set aside.
Add butter and chocolate chips to a microwave-safe bowl. Heat in the microwave in 30-seconds increments, stirring every time until melted and smooth. Set aside to cool.
In a large mixing bowl, add eggs, light brown sugar, granulated sugar, and vanilla extract. Beat using an electric mixer for 2-3 minutes.
Slowly add the cooled chocolate butter mixture. Whisk until combined. Stir in the dry ingredients until just combined using a rubber spatula. Don’t overmix.
Using a cookie scoop or a spoon, scoop about 1 tbsp of batter and place it on the prepared baking sheet, leaving about 2 inches of space between each cookie. Work in batches.
Bake in the preheated oven for 10-12 minutes or until the edges are set and the top is cracked. Remove from the oven and let cool on the baking sheet. Then, transfer to a cooling rack.
Prepare the peanut butter filling. In a mixing bowl, beat peanut butter, powdered sugar, butter, vanilla extract, and salt for 2-3 minutes until creamy. Then, beat in the heavy cream for 1-2 minutes until fluffy. You can add more or less amounts of heavy cream to reach the desired consistency of the filling.
Spread the peanut butter filling onto the flat side of the brownie cookies using a piping bag or a spoon. Top with the second brownie cookie to make a sandwich. Place in the refrigerator until ready to serve or eat immediately.