Preheat the oven to 350ºF (180°C). Line an 8-inch (20-cm) square baking pan with parchment paper.
Add the butter and chocolate to a large heat-safe bowl.
½ cup butter, 4 oz. bittersweet chocolate
Bring a small saucepan of water to a simmer over medium heat.
Place the bowl of chocolate and butter on top of the pot of water (the water shouldn't touch the bowl).
Let the steam from the water warm the underside of the bowl (double boiler-style) until the chocolate and butter are just melted. Don't overheat.
Once the chocolate mixture is melted, remove from the heat. Whisk in the eggs, then add the sugars, cocoa, baking powder, salt, and vanilla. Whisk again until smooth.
2 large eggs, ¾ cup white sugar, ¼ cup packed brown sugar, 2 tablespoons cocoa powder, ½ teaspoon baking powder, ¼ teaspoon sea salt, ½ teaspoon vanilla extract
Next, stir in the flour until the mixture is smooth, being careful not to over-mix.
½ cup all-purpose flour
Transfer the batter into the prepared tin. Drop teaspoon-fulls of peanut butter over the batter, then swirl with a knife.
¼ cup natural peanut butter
Bake for 23-25 minutes. They will seem slightly underdone. Let the brownies cool completely in the pan, then slice to serve. Once cool, store in the fridge.
Notes
If the peanut butter is a little hard or dry, gently heat until it's soft enough to swirl.