Preheat oven to 350 degrees F. Grease a 9x13 baking pan and line with parchment paper, set aside.
In a small bowl, whisk together the flour, salt &, baking powder.
In a large bowl, whisk together the eggs, sugars, oil and vanilla.
Place the chocolate and butter in a microwaveable bowl and melt chocolate mixture in 30 second increments until completely melted, approx 2 minutes. (Can also melt on stovetop over low heat , whisking frequently, until completely melted and smooth in a small heavy-bottom saucepan.
Add white chocolate mixture to the sugar-egg mixture and whisk until smooth. Add the flour mixture and stir with spatula until just combined.
Pour batter into prepared pan and bake for 28-32 minutes, until sides of brownies have set, the top is starting to crackle and look glossy. Brownies are done when the toothpick comes out with a few moist crumbs still clinging. It's okay for the pick to look moist, but if you see wet batter, keep baking.
Transfer pan to a wire rack and let cool completely.
Use parchment to lift out brownies.
Cut into bars and serve.
Store brownies in an airtight container at room temperature for up to 2 days.