Preheat your oven to 325*F and line a 9"x13" baking pan with parchment (or foil). Leave some paper hanging over the sides, so you can easily pull the blondies 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 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 mix-ins of your choice (chips, nuts, coconut) to the mixing bowl and blend just until evenly incorporated.

Spoon dough into prepared pan and spread with an off-set spatula until dough is spread evenly in pan.

Bake 35-40 minutes, or until the top is golden brown.

Remove from oven and allow to cool FULLY before cutting (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 blondies on a cutting board. Cut with a serrated knife, wiping off between cuts as needed.