Add the granulated sugar to a medium bowl. Add the zest of the entire lemon. Rub the lemon zest and sugars together to release the oils in the zest.
Slice the salted butter into 3 tablespoon pieces and place in a medium bowl. Finely chop the white chocolate and add to the same bowl.
Microwave for 30 seconds. Stir to mix and then microwave again for another 15-20 second intervals. Stop to stir between intervals until it is fully melted and combined. Set aside.
Add the egg, lemon sugar, lemon extract, vanilla extract to a medium bowl.
Beat with a mixer until the mixture turns pale. About 2-3 minutes.
Stir in the melted white chocolate butter mixture until fully combined.
Add the flour and mix until just combined.
Preheat your oven to 325°F. Line an 9x5-inch baking pan with parchment paper.
Add the batter to the pan. Gently tap the pan on the counter to remove any air bubbles.
Bake for 15-18 minutes, or until the top is lightly golden. Let cool in pan for at least 1 hour before adding the glaze. As soon as you add the glaze. Cool for at least another 30 minutes in the refrigerator to set the glaze. Then cut into squares.