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. Mix well and set aside.

In a large mixing bowl, add white sugar and lemon zest. Rub the lemon zest into the sugar until the sugar is light yellow.

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

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

Pour the batter into the prepared cake pan. Shake to distribute evenly and drop lightly on the counter twice.

Bake for 22 minutes, or until a toothpick comes out almost clean. Cool completely in the cake tin for 1-2 hours.

Once cooled, run a knife around the edges and lift the brownie out using 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 let it set for 30 minutes to 1 hour before slicing and serving.