Prepare the pan and oven: Heat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Melt the butter: In a microwave-safe bowl or small saucepan, melt the butter until just liquid, not sizzling. Let it cool for 2 minutes so it doesn’t scramble the eggs.

Whisk in sugars: Add granulated and brown sugar to the butter and whisk until glossy and combined, about 30 seconds.

Add eggs and vanilla: Whisk in eggs one at a time until the mixture thickens slightly and looks shiny. Stir in vanilla. This helps create that crinkly top.

Mix dry ingredients: In a separate bowl, whisk cocoa powder, flour, salt, and baking powder (if using). Sift if your cocoa is clumpy.

Combine wet and dry: Add the dry ingredients to the wet mixture. Stir gently with a spatula until just combined. Do not overmix.

Fold in chocolate: Stir in the chocolate chips or chopped chocolate. The batter will be thick.

Spread in pan: Scrape the batter into the prepared pan and smooth the top.

Loosen the swirls: In two small bowls, warm peanut butter and Nutella separately for 10–15 seconds in the microwave. Stir in a splash of milk or cream if needed to reach a spoonable, drizzle-ready consistency.

Create the swirl: Dollop peanut butter and Nutella over the brownie batter in alternating spoonfuls. Use a butter knife or skewer to gently swirl, dragging in figure-eight motions. Don’t overmix—you want defined ribbons.

Bake: Bake 22–28 minutes, depending on your oven and preferred texture. The edges should be set, and a toothpick inserted in the center should come out with moist crumbs, not wet batter.

Cool completely: Place the pan on a rack and let the brownies cool fully, at least 1 hour. For clean cuts, chill for 30 minutes before slicing.

Finish and serve: If using, sprinkle with flaky salt just after baking. Lift out, slice into 9 large or 16 smaller squares, and enjoy.