1. Preheat oven to 350°F. Line an 8 x 8 inch baking pan with parchment paper. Allow the paper to hang over the sides of the pan for easy removal of the brownies once baked. Set aside.
2. Melt the butter and chopped white chocolate in a double boiler over medium-low heat. To create a double boiler, fill a medium saucepan with about 2 inches of water. Place a heat-safe bowl over the top of the pan and add the butter and chocolate. Heat over medium-low heat, bringing the water to a simmer. Stir often to prevent the white chocolate from seizing while melting. Once melted, remove the bowl from the heat and set aside to cool for 15 to 20 minutes.
3. In a large bowl, add the granulated sugar and lemon zest. Use your hands to massage the zest into the sugar. This helps to release the oils from the lemon zest into the sugar, amping up the lemon flavor once the brownies are baked.
4. Add the eggs and egg yolk. Use a whisk to combine well.
5. Add the cooled butter and chocolate mixture and the vanilla. Whisk until well combined.
6. Add the flour and salt. Use a large spatula or wooden spoon to fold the wet and dry ingredients together. Stop when a few streaks of flour remain. Do not over mix.
7. Transfer the batter to the prepared pan and use a spatula to gently smooth the top.
8. Bake for approximately 25 minutes or until the edges are golden brown and the top is just barely set in the center. Do not over bake or the brownies will be dry.
9. Remove the pan from the oven and set on a wire rack. Cool the brownies completely before adding the lemon glaze.

For Glaze

1. To make the glaze, combine the powdered sugar and lemon juice (or milk) in a small bowl. Whisk until smooth but pourable. If the glaze is too thin, add more powdered sugar. If too thick, add more liquid.
2. Pour over the top of the cooled brownies and use an offset spatula to spread evenly.
3. Top with additional lemon zest, if desired.