Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper or lightly grease it, leaving some overhang for easy removal.

In a microwave-safe bowl, melt the chopped dark chocolate and butter in 20-second increments, stirring after each one until smooth and glossy. Set aside to cool slightly.

In a large mixing bowl, whisk together the granulated sugar, brown sugar, eggs, and vanilla extract until the mixture becomes pale and slightly fluffy—about 2 minutes.

Pour the cooled chocolate-butter mixture into the egg and sugar mixture, stirring until fully combined.

Over the wet mixture, sift in the flour, cocoa powder, salt, and espresso powder. Gently fold the dry ingredients in with a spatula, mixing just until no flour streaks remain.

Pour the rich, chocolatey batter into your prepared pan, smoothing the top with a spatula.

Drizzle the warmed creamy peanut butter in several horizontal lines across the batter’s surface. Using a thin knife or skewer, gently swirl the peanut butter into the brownie batter. Don’t over-mix—the goal is a rippled, marbled look.

Bake for 25-30 minutes, or until a toothpick inserted near the edge comes out mostly clean with a few moist crumbs attached. For extra-gooey centers, err on the shorter baking time.

Cool completely in the pan, then lift out using the parchment overhang. Slice into squares and enjoy!