Prep Your Baking Pan: Preheat your oven to 180°C (350°F). Grease and line an 8x8-inch baking pan with parchment paper, leaving extra paper for easy removal later.
Combine Dry Ingredients: Sift together flour, cornstarch, and salt in a bowl. Whisk well and set aside.
Infuse Sugar with Lemon Zest: In a large mixing bowl, rub lemon zest into granulated sugar to release its oils, creating a fragrant, citrusy sugar mixture.
Create the Wet Mixture: Add melted butter, vegetable oil, eggs, lemon juice, and vanilla to the sugar mixture. Whisk until smooth but don’t overmix.
Combine Wet and Dry Ingredients: Gradually fold the dry ingredients into the wet mixture, mixing just until combined to avoid overworking the batter.
Bake: Pour the batter into the prepared pan and smooth the top. Bake for 22–25 minutes or until a toothpick inserted comes out with a few moist crumbs.
Cool Completely: Let the brownies cool in the pan for 1–2 hours before lifting them out using the parchment paper.
Make the Glaze: Mix powdered sugar and lemon juice in a small bowl until smooth and spreadable.
Glaze and Set: Spread the glaze over the cooled brownies. Let it set for 30 minutes to an hour before slicing and serving.