Preheat the oven to 350°F (180°C), or 160°C for a fan oven. Line a 9x9-inch (approx 22x22 cm) baking pan with parchment paper and set aside.
In a large, microwave-safe bowl, melt the butter and 2 ounces (55 g) of the white chocolate in 30-second intervals, stirring after each, until melted and smooth.
Whisk in the granulated sugar, brown sugar, lemon zest, and salt until smooth.
Add the eggs and lemon juice, then whisk until fully combined.
Stir in the flour until no dry streaks remain.
Fold in the remaining chopped white chocolate until evenly distributed.
Transfer the batter to the prepared pan and smooth the top into an even layer.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Allow the brownies to cool completely in the pan before adding the glaze.