Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, allowing some overhang for easy removal.

In a large mixing bowl, whisk together the melted butter and sugar until smooth. Add eggs one at a time, beating well after each addition. Stir in vanilla extract.

In a separate bowl, combine the flour, cocoa powder, and salt. Gradually fold the dry ingredients into the wet ingredients until just combined.

Pour half of the brownie batter into the prepared pan, spreading evenly. Drop spoonfuls of cookie butter over the batter, then pour the remaining brownie batter on top. Use a knife or skewer to gently swirl the layers together.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Allow the brownies to cool in the pan for about 15 minutes, then lift them out using the parchment paper and let them cool completely on a wire rack.

Once cooled, use a sharp knife to cut the brownies into squares. Serve immediately or store as directed.

Store leftover brownies in an airtight container at room temperature for up to 4 days, or refrigerate for up to a week. For longer storage, freeze in a freezer-safe bag for up to 3 months.