Preheat the oven to 325°F. Line an 8x8 inch baking tin with parchment paper.

In a large mixing bowl, combine the brownie mix, water, egg, and avocado oil. Use an electric mixer or whisk to blend until the batter is smooth and well-combined.

Pour the brownie batter into the prepared baking tin, spreading it evenly.

Drizzle the peanut butter over the batter. You can use 2 to 4 tablespoons, depending on your preference, and swirl it into the batter. For extra creaminess, feel free to add a little more peanut butter.

Bake in the preheated oven for 15-18 minutes, or until a toothpick inserted into the center comes out clean.

Allow the brownies to cool on a cooling rack before cutting them into squares. Enjoy!