Preheat your oven to 180°C (350°F) and prepare an 8x8 inch square cake tin.
Sift together the all-purpose flour, cornstarch, and salt in a mixing bowl and set aside.
Rub lemon zest into the white granulated sugar for about 30 seconds until aromatic.
Add warm melted unsalted butter, unflavored vegetable oil, large eggs, freshly squeezed lemon juice, and vanilla extract to the lemon sugar mixture and whisk gently.
Fold the dry mixture into the wet ingredients gently.
Pour the batter into the prepared cake tin and shake gently to distribute evenly.
Bake for about 22 minutes and check for doneness with a toothpick.
Allow the brownies to cool completely in the pan for 1 to 2 hours.
Lift the brownies using parchment paper and place them on a wire rack to cool completely.
To make the zesty lemon glaze, whisk powdered sugar with freshly squeezed lemon juice until smooth.
Spread the glaze over the cooled brownies and let it set for 30 minutes to an hour before slicing.