Preheat your oven to 350°F (175°C) and grease an 8x8-inch baking pan or line it with parchment paper. In a medium bowl, whisk together the flour, baking soda, salt, and cocoa powder.

In a large bowl, combine the granulated sugar, brown sugar, and melted butter. Mix until well combined, then add the eggs and vanilla extract, stirring until smooth. Gradually add the dry ingredients, stirring gently until just combined. Fold in the semi-sweet chocolate chips.

Pour the brownie batter into the prepared baking pan and spread evenly. Bake for 25-30 minutes, checking for doneness with a toothpick.

While the brownies are baking, mix together the peanut butter and powdered sugar until smooth.

Once the brownies are baked, let them cool for 15 minutes. Drop spoonfuls of the peanut butter mixture over the brownie layer and swirl gently with a knife.

Sprinkle mini chocolate chips on top and allow the brownies to cool completely. Slice into squares and chill in the refrigerator before serving for best flavor.