Preheat your oven to 350°F (175°C) to ensure even baking.
Line a 9x13 inch baking pan with parchment paper, leaving overhang for easy removal.
In a medium saucepan over low heat, melt 1 cup of butter. Remove from heat and let it cool slightly.
In a large mixing bowl, whisk together the melted butter and 2 cups of sugar until well combined. Add in 3 eggs and 1 tablespoon of vanilla extract, mixing until smooth.
Sift in 1 cup of cocoa powder, 1 cup of flour, ½ teaspoon of salt, and ½ teaspoon of baking powder. Stir gently until just combined; be careful not to overmix.
In a separate bowl, mix together 1 cup of creamy peanut butter, 1 cup of powdered sugar, and 1 teaspoon of vanilla extract until smooth and creamy.
Pour half of the brownie batter into the prepared baking pan, spreading it evenly. Dollop the peanut butter truffle filling over the brownie layer, then pour the remaining brownie batter on top, smoothing it out.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the brownies from the oven and let them cool in the pan for about 15 minutes. Use the parchment paper to lift them out onto a cooling rack to cool completely before cutting into squares.
