Preheat oven to 350°F/175°C. Grease a 9×13 inch pan and line with parchment paper.
In a medium bowl whisk the flour and baking powder together
In a small saucepan, melt the butter. Let the butter begin to brown, once it gives off a nutty, toasty smell with brown flecks at the bottom, take it off the heat. Add the sugars, vanilla, coffee and salt and combine. Let it cool to room temperature.
Add the eggs and whisk until combined.
Pour the butter mixture into a large bowl and add the flour mixture. Stir until just combined – don't overmix it!
In a small bowl, microwave the peanut butter and butter together until smooth and creamy.
Pour the batter into the prepared pan. Drop 5 large dollops of the peanut butter filling and use a butter knife to swirl it around the top.
Bake for 22-24 minutes, until the edges are set and the top is golden. When you insert a toothpick, you should still have a few crumbs. We don't want to overbake it!
Transfer to a wire rack and let it cool completely. Cut into bars.
Mocha Drizzle
Melt dark chocolate in a microwave in 15 second bursts
Add icing sugar, water and espresso powder and mix until smooth.