1cup plus 2 tbsp (225 g)lightly packed brown sugar
1cup plus 2 tbsp (225 g)granulated sugar
4largeeggs, at room temperature
1cup (170 g)semi sweet chocolate chips
Peanut Butter Swirl
½cup (1 stick or 113 g)butter, melted
1½cups (360 g)smooth peanut butter
1cup (125 g)powdered sugar
Make the Brownie Batter
Preheat the oven to 350° F. Line a 7 by 11-inch baking pan with parchment paper, or grease the bottom and sides.
In a large bowl, whisk together the melted butter, oil, and both sugars. Add the eggs, vanilla, and salt and continue to whisk until completely combined and light in color.
In a separate bowl, or on a piece of waxed or parchment paper, sift together the flour and unsweetened cocoa powder. Add to the rest of the ingredients in the bowl and carefully mix until fully incorporated. The batter will be thick.
Scrape the batter into prepared pan and smooth the top. Set aside.
Make the Peanut Butter Swirl
In a large bowl, stir together the peanut butter, butter, powdered sugar, and vanilla with a large spoon or spatula until smooth and creamy with no lumps.
Dollop large spoonfuls all over the top of the brownie batter and swirl back and forth with a butter knife or spoon.
Bake for 40-45 minutes, or until a toothpick or long skewer inserted into the center comes out mostly clean.
Allow to cool in pan on wire rack before serving. For uniform slicing, refrigerate until firm before cutting into squares.