Preheat oven to 350°F. Line a 9x9 (or 10x10) pan with parchment.
In a large bowl, whisk butter, sugar, and lemon zest until light and fluffy.
Add egg, yolk, lemon juice, lemon extract, and vanilla. Stir well.
Add flour, cornstarch, baking soda, and salt. Mix until just combined.
Spread into pan and bake 28–32 minutes, until a toothpick comes out clean or with a few moist crumbs. Cool completely.
Whisk glaze ingredients until smooth. Pour over cooled brownies and let set for 20 minutes (or chill for 10).
Slice and devour.
