Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper. This helps the cookies keep their shape and prevents sticking.

Melt the chocolate and butter together. Place them in a heatproof bowl and microwave in 20–30 second bursts, stirring between each until smooth. Let cool for 5 minutes so the mixture is warm, not hot.

Whisk the eggs and sugars. In a large bowl, whisk the eggs, granulated sugar, and brown sugar for 1–2 minutes until slightly thick and lighter in color. This step helps create that shiny, crackly top.

Add vanilla and melted chocolate. Whisk until fully combined and glossy.

Combine the dry ingredients. Sift in the flour, cocoa powder, baking powder, and salt. Fold gently with a spatula until just combined. The batter should be thick and shiny, like brownie batter. Stir in extra chocolate chips if using.

Warm the peanut butter slightly. Microwave for 10–15 seconds to make it easier to swirl. It should be fluid but not hot.

Scoop the dough. Use a medium cookie scoop (about 2 tablespoons) and place mounds on the prepared sheets, leaving 2 inches between cookies.

Swirl the magic. Spoon roughly 1/2 teaspoon of warm peanut butter on top of each dough mound. Use a toothpick or the tip of a knife to swirl it gently into the surface. Don’t overmix; visible ribbons look and taste best.

Bake for 9–11 minutes. The cookies are done when the edges are set, the tops are glossy with thin cracks, and the centers still look slightly soft. They will firm up as they cool.

Finish and cool. If you like, sprinkle a pinch of flaky sea salt over the warm cookies. Let them cool on the sheet for 5 minutes, then transfer to a rack to cool completely—or enjoy warm and gooey.