Preheat oven to 180 ?C (350?F) and grease an 8x8 inch cake tin. Line with parchment paper, leaving overhang on two opposite edges.

Sift together flour, cornstarch, and salt. Mix and set aside.

In a large mixing bowl, combine white sugar and fresh lemon zest. Rub the lemon zest into the sugar until it has a light yellow tint.

Add warm melted butter, unflavored vegetable oil, eggs, lemon juice, and vanilla to the mixture. Whisk until smooth, avoiding overmixing.

Gently fold the dry ingredients into the wet mixture until just combined.

Pour the batter into the prepared cake pan and tap lightly on the counter to evenly distribute. Bake for 22 minutes, or until a toothpick comes out almost clean.

Allow cooling in the cake tin for 1-2 hours.

Once cooled, run a knife around the edges and lift the brownie out using the parchment paper.

Prepare the lemon glaze by mixing powdered sugar and lemon juice until smooth and thick but spreadable.

Spread the glaze evenly over the brownie and allow to set for 30 minutes to an hour before slicing and serving.