Preheat oven to 325°F. Use baking nonstick spray on a 9x9-inch square pan. Place parchment paper on the pan's bottom and sides. Spray again.
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In a small sauce pan, place butter (cubed) and white chocolate. Fully melt over medium-low heat. Remove heat.
2
Sift flour, cornstarch, and salt in a medium basin. Set aside.Mix sugar, eggs, and egg yolk on high for 5 minutes. Mix oil, vanilla, lemon zest, and juice with butter/white chocolate mixer. Blend barely.
3
Add dry ingredients and mix slowly. Disconnect the mixer and scrape the bowl with a rubber spatula. Pour batter into pan.
4
Bake until a toothpick inserted yields moist brownie crumbs, 40–45 minutes. If there's a lot of raw batter, bake for 2 more minutes and check.Cool the heated pan on a rack. Cool fully.
5
In a small dish, combine powdered sugar and lemon juice. Cover brownies with frosting. Cut brownies once frosting sets.
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