Preheat the oven to 350° F. Line an 8x8 baking pan with parchment paper, leaving an overhang for easy removal. Set aside.
In a large microwave-safe bowl, melt the butter and semi-sweet chocolate for 30 seconds, and then again in 10-second increments (mixing after each time) until completely melted and smooth.
Add the sugar to this bowl and whisk until fully combined. Add in the eggs + yolk, making sure to mix fully so that the eggs are incorporated into the batter. Then add the vanilla.
Add the flour, cocoa powder, and salt, straight into the bowl. Mix until everything is just combined.
Gently fold in the white and dark chocolate using a rubber spatula.
Pour batter into the baking pan, spread out evenly, and top with a few extra white and dark chocolate chips (optional).
Bake in the oven for 30-32 minutes (until the center of the brownies are set or a toothpick comes out clean). Let the brownies cool for at least 30 minutes before cutting and serving.
Notes
I recommend using Ghirardelli chocolate. I like both their bars and chips.
Use a bit of arm power and really whisk those eggs into the sugar mixture, this will give you those shiny crinkle tops on your brownie.
For fudgy, rich brownies you want them to be just the slightest bit underbaked, they continue baking in the pan once you take them out.
Let me cool a bit before cutting into them! Cutting warm brownies won't give you nice slices.