Preheat & Prep: Preheat your oven to 350°F (175°C). Grease an 8×8-inch square baking pan or a loaf pan, then line it with parchment paper for easy removal.
Melt & Mix: In a microwave-safe bowl, melt the butter. Whisk in vegetable oil, granulated sugar, brown sugar, and vanilla extract until smooth.
Incorporate Eggs: Add the egg and whisk until fully combined, then mix in the egg yolk.
Add Flour: Using a spatula, gently fold in the all-purpose flour until fully incorporated.
Fold in Oreos: Chop the peanut butter Oreos into chunks and fold them into the batter.
Spread & Swirl: Pour the batter into the prepared baking pan and spread it evenly. Dollop the creamy peanut butter on top and use a knife or toothpick to create swirls.
Bake: Place in the oven and bake for 18-20 minutes for a square pan or 28-30 minutes for a loaf pan. The blondies should have a slightly soft center but set edges.
Cool & Enjoy: Let the blondies cool completely before cutting. If you cut them too soon, they may break apart—but we won’t blame you if you sneak a bite while they’re warm!