Preheat your oven to 325*F and line a baking pan with parchment (or foil). Leave some paper hanging over the sides, so you can easily pull the bars out of the pan after baking. Lightly butter any portion of the pan that is not covered in paper (this should be the two shorter sides of the pan)
In a large mixer, blend together both sugars and warm, melted butter. Blend until well combined, then add peanut butter, eggs (one at a time) and vanilla and continue to blend until fully combined.
In a separate bowl whisk together flour, baking powder and salt. Then, while the mixer is on low, slowly add the flour to the butter/sugar mixture. Stop the mixer immediately when you have added all the flour.
Add chocolate chips (or chips of your choice) and blend until just combined.
Scoop dough into prepared pan and spread with an off-set spatula (the surface should be level before baking).
Bake 35-40 minutes, or until the top is golden brown and there is no wet batter visible.
Remove from oven and allow to cool FULLY before cutter (1 hour or more). For easiest cutting, run a knife around any edges of the bars that are touching the sides of the pan, then pull up on the parchment paper and place the giant bar on a cutter board. Cut with a serrated knife, wiping off between cuts as needed.