Preheat the oven to 350°F. Line an 8x8-inch square baking pan with a parchment paper sling.
Add the butter, oil, sugar, eggs, egg yolk, lemon zest, lemon juice, and vanilla extract to a large mixing bowl. Whisk well to combine
¼ cup salted butter, ¼ cup vegetable oil, ¾ cup (150g) granulated sugar, 2 large eggs, 1 tablespoon lemon zest, 3 tablespoons fresh lemon juice, 1 teaspoon vanilla extract
Add the flour, cornstarch, baking powder, and salt. Stir just until combined. Stir in the white chocolate chips.
1 cup (141g) all-purpose flour, 2 teaspoons cornstarch, ¼ teaspoon baking powder, ¼ teaspoon salt, 1 cup white chocolate chips
Pour the mixture into the prepared baking dish and place in the oven and bake for 18–20 minutes. Remove from the oven and allow the brownies to cool for about a half hour.
While the brownies are cooling, create the glaze by combining the ingredients in a small bowl until smooth. Glaze the top of the brownies once they've cooled, then refrigerate for 1-2 hours until the glaze is set and the bars are chilled.
1 cup powdered sugar, 1 tablespoon fresh lemon juice, 1 teaspoon lemon zest