Preheat the oven to 350F (180C).
Lightly grease an 8x8 inch (20x20 cm) and then line with parchment paper, leaving an overhang around the edges. Greasing the pan first helps the parchment paper stay in place. Alternatively, you can simply grease the pan.
In a large bowl, mix together the melted butter, brown sugar and peanut butter until smooth. This can be done with a wire whisk or an electric mixer.
Mix in the eggs and vanilla extract until combined.
Gently mix in the flour and salt. Stop mixing as soon as you no longer see clumps of flour.
Spread about ½ of the mixture into the bottom of the prepared pan.
Place a layer of Oreos on top of the peanut butter dough and press them down slightly. I used 4 rows of 4 Oreos.
Drop spoonfulls of the remaining peanut butter dough over the Oreo cookies so that the dough is evenly distributed across the pan.
Spread the dough smooth using a flat edge knife so that the cookies are completely covered.
Bake in the preheated oven for about 23-27 minutes, or until the top looks set and an inserted toothpick comes out clean or with a few damp crumbs. Note that bake times will vary depending on your oven and how gooey you like your blondies.
For clean cuts, let the bars cool completely before slicing. When ready to slice, lift the cooled bars out of the pan using the overhang of the parchment paper. Place on a cutting board and slice using a sharp, think knife.