Preheat oven to 350°F/180°C/Gas 4. Line an 8” x 8” square pan with parchment paper and spray with non-stick cooking spray. Set it aside.
For the peanut butter layer, place the peanut butter, butter, sugars, egg, and vanilla into a mixing bowl and stir to combine. Add the flour, baking powder, and salt, and lightly fold into the mixed wet ingredients until just combined. TIP! Do not over-mix as the brownies will turn out cake-like.
Spread the peanut butter mixture into the prepared baking pan and spread the batter out evenly.
For the chocolate layer, place the butter, sugar, eggs, and vanilla into a mixing bowl and stir to combine. Add the flour, cocoa powder, and salt and as before, lightly fold into the mixed wet ingredients until just combined.
Spread the chocolate brownie layer on top of the peanut butter layer.
Bake for 22 minutes in the preheated oven. The brownies will still be soft and gooey but will finish setting while they cool. When completely cool, slice into 9 pieces and enjoy!