In a large mixing bowl, beat together creamy peanut butter, softened unsalted butter, and brown sugar until light and fluffy, about 2-3 minutes. Then, add in egg and vanilla extract, mixing until fully combined. Gradually incorporate all-purpose flour, baking soda, and salt until a soft dough forms. Chill this mixture in the refrigerator for 30 minutes.

In a separate bowl, melt unsalted butter and mix in granulated sugar and Dutch-processed cocoa powder until smooth. Allow it to cool slightly before whisking in egg and an additional teaspoon of vanilla extract for flavor. Gradually add in all-purpose flour and chocolate chips, stirring just until combined. Chill the brownie dough in the fridge for another 30 minutes.

While the doughs chill, preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper to prevent sticking.

Once the doughs are chilled, scoop rounded tablespoons of each dough onto the prepared baking sheets, alternating the peanut butter and brownie dough. Leave about 2 inches of space between each scoop, as they will spread while baking. Use a toothpick to gently swirl the two doughs together, creating a marbled effect.

Place the baking sheets in the preheated oven and bake the cookies for 9-11 minutes, or until the edges are set and the centers appear slightly soft. It's crucial not to overbake them.

After baking, let the cookies cool on the baking sheet for about 5 minutes before transferring them to wire racks. Allow the cookies to cool completely before serving.