Preheat the oven to 350F, grease and line a 9x13 inch baking sheet with parchment paper.

In a small saucepan over low heat, melt the butter and chocolate chunks. Sift the cocoa powder and combine, whisking until glossy.

Sift flour, whisk in the salt and set aside.

In the bowl of a stand mixer with the whisk attachment, whisk eggs and sugar until creamy. About 5 minutes. Switch to the paddle attachment and add the chocolate mixture and oil, and continue beating.

Fold in the flour using a spatula, until fully combined.

Pour onto lined baking sheet.

For the peanut butter swirl: place peanut butter in a heat-proof bowl and microwave for 30 seconds. Whisk in powdered sugar until smooth.

Spoon and drop peanut butter onto batter and make swirls using a knife or spatula.

Bake for 30 minutes, until the top forms a crust. Peanut butter will have a soft wobble.