Prep Your Pan & Oven: Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving some overhang on the sides for easy lifting later. Set it aside.
Melt Chocolate & Butter: Set up a double boiler: fill a medium saucepan with about 2 inches of water and place a heat-safe bowl on top. Add the butter and chopped white chocolate to the bowl. Heat over medium-low, stirring frequently, until completely melted and smooth. Remove from heat and let it cool for 15-20 minutes. This cooling step helps create perfect brownies!
Boost Lemon Flavor: In a large mixing bowl, combine the granulated sugar and lemon zest. Use your hands to gently massage the zest into the sugar for about 20 seconds. This simple trick releases more lemon oil, making your brownies extra fragrant and flavorful.
Whisk in Eggs: Add the 3 whole eggs and the extra egg yolk to the lemon-sugar mixture. Whisk them together until everything is well combined and smooth.
Combine Wet Ingredients: Pour the cooled butter and white chocolate mixture, along with the vanilla extract, into the egg and sugar bowl. Whisk until it's fully blended.
Fold in Dry Ingredients: Add the flour and salt to the bowl. Using a large spatula or wooden spoon, gently fold the dry ingredients into the wet mixture. Stop mixing as soon as a few streaks of flour remain; overmixing leads to dry brownies.
Bake to Perfection: Transfer the brownie batter to your prepared pan and spread the top evenly with a spatula. Bake on the center rack for about 25 minutes. The edges should be golden brown, and the center should be just barely set (it shouldn't jiggle much when you gently shake the pan). Avoid overbaking for fudgy results!
Cool Completely: Remove the pan from the oven and place it on a wire rack. Let the brownies cool completely in the pan for at least 2 hours before adding the glaze. This is crucial for easy slicing and a perfect texture.
Make the Glaze: While the brownies cool, whisk together the sifted powdered sugar and fresh lemon juice (start with 4 tablespoons, add more if needed) in a small bowl until smooth and pourable.
Glaze & Serve: Once the brownies are fully cooled, pour the glaze over them and spread it evenly with an offset spatula. You can sprinkle extra lemon zest on top if you like! Let the glaze set for a bit, then lift the brownies out of the pan using the parchment paper overhang, slice into squares, and enjoy your delightful lemon treats!