Start by letting the butter soften at room temperature for about an hour – you should be able to easily press your finger into it, but it shouldn’t be melty. Preheat your oven to 350°F and line two baking sheets with parchment paper.

In a large bowl, beat the softened butter and peanut butter together until they’re completely smooth and well-combined, about 2 minutes. You’ll want to scrape down the sides of the bowl at least once. Add both sugars and cream until the mixture looks light and fluffy – this usually takes about 3 minutes and the color will become noticeably lighter.

Beat in 2 eggs, one at a time, mixing until each one disappears completely into the batter. If you see any streaks of egg, keep mixing.

In a separate bowl, whisk together your flour, baking powder, salt, and baking soda. Add this dry mixture to your peanut butter mixture in three portions, mixing on low speed just until you don’t see any dry flour. The dough will be pretty thick at this point.

Now for the brownie batter: In another bowl, combine the brownie mix, vegetable oil, 2 eggs, and water. Mix until well combined – it should be thick but still fluid enough to drop from a spoon.

Here’s where it gets fun: Take about 2 tablespoons of peanut butter dough and roll it into a ball. Make a thumbprint-sized indent in the center. Drop about 1 teaspoon of brownie batter into the indent, then take another small piece of peanut butter dough and partially cover the brownie batter. Don’t worry about making it perfect – the messier the better for a nice swirl effect.

Place the cookies about 2 inches apart on your prepared baking sheets. These will spread a bit while baking.

Bake for 12-14 minutes, rotating the pan halfway through. You’re looking for edges that are just starting to turn golden brown, while the centers should still look slightly soft. If you tap the edge of a cookie lightly, it should feel set but not hard.

Let them cool on the baking sheets for 5 minutes – they’ll be too soft to move right away. Then transfer them to a wire rack to cool completely. The brownie swirl will firm up as they cool but stay slightly fudgy.