In a large bowl, beat together peanut butter, softened butter, granulated sugar, and brown sugar until creamy and fluffy.

Add the egg and vanilla, mixing well.

In a separate bowl, whisk flour, baking soda, and salt.

Add dry ingredients to the peanut butter mixture and combine until a soft dough forms.

Chill in the refrigerator for 30 minutes.

In a microwave-safe bowl, melt the chocolate chips and butter in 30-second intervals, stirring until smooth.

Set aside to cool slightly.

In another bowl, whisk together the sugar, egg, and vanilla.

Stir this mixture into the melted chocolate.

Sift in the flour, cocoa powder, and salt, mixing gently until thick and glossy.

Preheat the oven to 350°F (175°C).

Line a baking tray with parchment paper.

Scoop a small ball of peanut butter dough and an equal-sized ball of brownie batter.

Lightly flatten each ball.

Press the two doughs together, twisting slightly to create a marbled pattern.

Roll into a smooth ball if needed, then place on the tray with at least 2 inches of space between each cookie.

Bake for 8–10 minutes, just until the edges are firm but the centers still look soft.

Cool on the tray for 5 minutes before transferring to a wire rack.

As they cool, the swirl becomes even more defined and the texture sets perfectly.