4Tablespoons(58g) unsalted buttersoftened to room temperature
1/4cup(25g) granulated sugar
6Tablespoons(96g) peanut butternot natural
Preheat the oven to 350ºF (177ºC). Line an 8" or 9" square baking pan with foil and spray with nonstick spray. Set aside.
In a medium saucepan over medium heat, melt the butter and chopped chocolate, stirring frequently until completely smooth. Allow mixture to cool while you move onto the peanut butter cookie batter.
PEANUT BUTTER COOKIE LAYER
In a medium size bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, brown sugar, and granulated sugar on medium speed until light and fluffy (approximately 3 minutes). Add the peanut butter and beat again on medium speed until combined, scraping down the sides and bottom of the bowl as necessary. Add the egg and vanilla and beat again until completely combined.
Turn the speed down to low and slowly add the flour mixture until dough forms. Set aside and move onto finishing the brownie batter.
FINISH THE BROWNIE LAYER
Whisk sugar into cooled chocolate mixture until combined. Add the eggs one at a time, whisking after each addition until smooth. Whisk in the vanilla.
Add the flour and salt and stir gently until completely combined. Set aside.
Spread 1 cup of the peanut butter cookie batter into the bottom of the prepared pan. I like to put 1/4 cup of batter in each corner and then gently spread it around with a spatula. Be gentle and patient.
Pour the brownie batter into the prepared pan right on top of the peanut butter cookie base, then top the brownie batter with the remaining peanut butter cookie dough in Tablespoon portions (you should have 6-7 Tablespoons left).
Using a butter knife, gently swirl the cookie dough around in the brownie batter. The batter will have some resistance and drag because of the texture of the cookie dough, but keep swirling.
Bake brownies for 32-35 minutes or until a toothpick inserted in the center comes out mostly clean. Allow brownies to cool completely before cutting into squares.
When you are ready to cut the brownies, remove the brownies by lifting the foil overhang straight up. Peel back remaining foil and place brownie block onto a large cutting board. Store leftover brownies at room temperature up to 1 week. Brownies freeze well, up to 3 months. Thaw in refrigerator overnight.
*Room temperature eggs incorporate into batter much more easily than cold eggs, although this is not absolutely necessary.