Preheat oven to 325°F. Spray a 9X9-inch square pan with baking nonstick spray. Line the bottom and 2 sides of the pan with parchment paper. Spray again.
In a small sauce pan, add the butter, (cut it into cubes first) and the white chocolate. Heat on medium-low heat until completely melted. Take off the heat.
4 oz White chocolate bar, ½ cup Unsalted butter
In a medium bowl, sift the flour, cornstarch, and salt. Set aside.
2 cups All-purpose flour, 1 tablespoon Cornstarch, 1 teaspoon Salt
Using a mixer, beat the sugar, eggs, and egg yolk on high-speed for 5 minutes. Add in the butter/ white chocolate mixer, oil, vanilla, lemon zest, and lemon juice. Mix on low until just combined.
2 cups White granulated sugar, 3 Large eggs, 1 Egg yolk, ½ cup Oil, 1 teaspoon Pure vanilla extract, 3 Lemons zested, 2 tablespoon Lemon juice
Add in the dry ingredients and mix on low speed. Take off the mixer and scrape the bowl with a rubber spatula. Pour the batter into the pan.
Bake for 40-45 minutes until a toothpick inserted comes out with moist brownie crumbs. If it is a lot of raw batter, bake for another 2 minutes and check again.
Put the hot pan on a cooling rack. Cool completely