Preheat the oven to 350°F. Line an 8 x 8 baking pan with aluminum foil or parchment paper.
Add the butter, chopped chocolate, granulated sugar, and brown sugar to a saucepan and heat on medium until the butter and chocolate are mostly melted. Stir constantly. Once they are mostly melted, remove from heat and stir until fully melted and set aside to cool slightly.
Beat in the eggs and vanilla and whisk vigorously for about 2 - 3 minutes until it is well-combined.
Add in the flour, cocoa powder, and salt. Mix until just combined.
Pour the batter into your prepared baking pan. Insert the mini peanut butter cups if you are using, then dollop the peanut butter over the top and use a knife to swirl it into the batter.
Bake for 30 - 35 minutes, up to 40, or until a toothpick comes out with very moist, thick crumbs. Cover with aluminum foil if the peanut butter seems to be browning too quickly.
Remove and allow to cool completely in the baking pan before removing and cutting.
Notes
Be careful to measure you dry ingredients carefully. Use a kitchen scale, if possible. Use room temperature eggs. You can quickly bring them to room temperature by leaving them in a bowl of warm water for five minutes. Do not swirl the peanut butter too much otherwise you will end up simply mixing the peanut butter into the batter.