Begin by preheating your oven to 180°C (350°F) and greasing an 8x8 inch square cake tin. For best results, line the tin with parchment paper, allowing some overhang to make it easier to lift out the brownies later.

In a medium bowl, sift together the all-purpose flour, cornstarch, and salt, then set it aside.

Add the fresh lemon zest to the granulated sugar, rubbing them together with your fingertips for about 30 seconds until the sugar takes on a light yellow hue.

Into the bowl with the sugar mixture, pour in the warm melted butter, vegetable oil, eggs, freshly squeezed lemon juice, and vanilla extract. Whisk everything together until smooth.

Gently fold in the dry ingredient mixture from Step 2.

Pour the brownie batter into your prepared tin, spreading it evenly with a spatula.

Place the tin in your preheated oven and bake for about 22 minutes.

Once baked, allow the brownies to cool completely in the tin for 1 to 2 hours.

To prepare the zesty lemon glaze, mix the powdered sugar with additional lemon juice in a small bowl until smooth.

Spread the lemon glaze evenly over the cooled brownies.

Finally, slice your Lemon Brownie Bars into squares and serve!