Preheat oven to 350F (175C) and grease and flour a 9x9 baking pan or line with parchment paper. Set aside.

In a large bowl, combine melted butter, sugar, and lemon zest. Stir well.

Add egg and egg yolk, lemon juice, lemon extract, and vanilla extract. Stir again until well-combined.

In a separate, medium-sized bowl, whisk together flour, cornstarch, baking soda, and salt.

Stir dry ingredients into wet until completely combined. Pour into prepared 9x9 pan and transfer to 350F (175C) oven. Bake 30 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Allow to cool completely before covering with glaze.

LEMON GLAZEOnce lemon brownies have cooled, prepare glaze by whisking together powdered sugar, 1 tablespoon lemon juice, and lemon zest until well-combined. If mixture is too stiff, add remaining lemon juice, 1 teaspoon at a time, until desired consistency is reached (when you lift a spoon or whisk out of the mixture, the glaze that drizzles off the spoon and falls back into the bowl should hold its ribbon shape for a few seconds before fading back into the rest of the glaze).

Pour glaze over lemon brownies, use a spatula to smooth evenly over the surface.

Allow glaze to harden (about 15-30 minutes) before slicing and serving lemon brownies.