Line a 9x13 inch (22 x 33 cm) pan with parchment so the bottom and sides are covered.
Peanut Butter Cookie Layer
In a large bowl beat together the butter, peanut butter and sugar.
Beat in the eggs and vanilla extract.
With the mixer on low speed, beat in the flour.
Spoon the mixture into the prepared 9x13 inch pan and spread smooth so that the bottom is covered. Use your hands to press it down as needed. You can use a lightly greased piece of wax paper to make this easier if needed.
Place the Oreo cookies on top of the peanut butter cookie dough, making 6 rows of 4 cookies.
Add the butter and chopped chocolate to a large heatproof bowl.
Microwave on medium power for 45 second intervals, stirring between each interval, until smooth.
Cool for a few minutes, then whisk in the sugar followed by the eggs.
Gently fold in the cocoa powder, flour, and salt.
Pour the brownie batter over the Oreo cookie layer.
Baking and Cooling
Bake in the preheated oven for 50-55 minutes, or until an inserted toothpick comes out with a few damp crumbs. I typically recommend checking around 45 minutes just in case.*
Remove from the oven and cool the brownies in the pan for at least 4 hours, or until completely cooled.
Remove from the pan by lifting up the sides of the parchment paper. Place on a cutting board and slice.
Light brown sugar can be substituted for 3/4 cup white sugar and 3/4 cup dark brown sugar.Semi-sweet chocolate can be substituted for 50-70% dark chocolate for richer brownies. Do not substitute with bittersweet. Semi-sweet or dark chocolate chips can be used if needed. If it looks like the top of the brownies are going to burn but the middle isn't finished baking, place a piece of aluminum foil overtop and continue baking.Store leftovers in an airtight container for up to 5 days.