Preheat oven to 180 °C (350°F) and grease a square 8x8 inch cake tin. Line with parchment paper.

Sift together flour, cornstarch, and salt. Set aside.

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

Add warm melted butter, unflavored vegetable oil, eggs, lemon juice, and vanilla to the bowl. Whisk until combined and smooth.

Add the dry ingredients and gently fold until just combined.

Pour the batter into the prepared cake pan, shake slightly to distribute evenly, and bake for 22 minutes or until a toothpick comes out almost clean.

Allow to cool completely 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.

Spread the glaze over the top of the brownie and allow to set for 30 minutes to 1 hour before slicing and serving.