Preheat Oven: Set your oven to 350°F. If you’re using a convection oven or if your oven runs hot, set it to 325°F. Grease and line an 8×8-inch square pan with parchment paper. Avoid using a glass pan.
Melt Butter and Chocolate: In a large bowl, combine the unsalted butter and chopped white chocolate. Melt them together by microwaving in 15-30 second intervals, stirring in between until fully melted. Alternatively, use a double boiler by placing the bowl over a pan of simmering water and whisking until melted.
Mix Wet Ingredients: Add the granulated sugar, eggs, and egg yolk to the melted chocolate mixture. Mix with a whisk or use a mixer on medium-high speed for about 4 minutes until the batter is light in color. Stir in the vanilla extract.
Add Dry Ingredients: Add the flour and salt to the bowl. Mix on low speed or fold with a spatula until combined.
Bake: Pour the batter into the prepared pan and smooth it out with a spatula. Bake for about 35 minutes, or until the edges are set and a toothpick inserted into the center comes out clean. If the top is browning too quickly but the center isn’t done, cover with aluminum foil and continue baking.
Cool: Let the brownies cool in the pan.