Line a 9x9-inch baking pan with parchment paper, leaving some overhang on the sides for easy removal.

In a medium mixing bowl, combine the melted butter, peanut butter, vanilla extract, and powdered sugar. Stir until a thick, smooth mixture forms.

Press the peanut butter mixture evenly into the prepared pan.

Place the pan in the freezer while you prepare the brownie batter.

Preheat your oven to 350°F (177°C).

In a large mixing bowl, whisk together the melted butter and chocolate chips until the chocolate chips begin to melt.

Add the eggs, egg yolks, and granulated sugar. Whisk until well combined and glossy.

Stir in the sourdough discard until fully incorporated.

Remove the peanut butter layer from the freezer.

Using the parchment paper, lift the frozen peanut butter layer out of the pan and set aside.

Re-line the same 9x9-inch pan with fresh parchment paper if needed.

Pour half of the brownie batter into the prepared pan and spread it into an even layer.

Carefully place the frozen peanut butter layer on top of the brownie batter.

Pour the remaining brownie batter over the peanut butter layer and gently spread it to completely cover the filling.

Bake for 38–43 minutes, or until the edges are set and a toothpick inserted into the brownie portion comes out with a few moist crumbs attached.

Allow the brownies to cool completely in the pan before slicing.