Preparation Steps

• Heat the oven to 350°F. Line an 8×8-inch metal pan with parchment, leaving a little overhang on two sides for easy lifting.
• In a small bowl, whisk together the flour, salt, and baking powder. Set aside.
• Place the butter and chopped white chocolate in a small saucepan over the lowest heat. Stir constantly until melted and smooth, then remove from heat and let cool 3–5 minutes.
• In a large mixing bowl, rub the lemon zest into the granulated sugar with your fingertips until the sugar is fragrant and slightly damp.
• Whisk the eggs and egg yolk into the lemon sugar for about 1 minute until slightly lighter in color. Whisk in the vanilla and lemon extract.
• Slowly stream the lukewarm white-chocolate mixture into the egg mixture, whisking constantly until glossy and combined.
• Switch to a spatula. Fold in the dry ingredients in two additions just until no dry streaks remain. Gently stir in the sour cream and then the lemon juice until evenly incorporated; do not overmix.
• Scrape the thick batter into the prepared pan and smooth the top. Tap the pan once on the counter to release large air bubbles.
• Bake 24–28 minutes (about 27 minutes is typical) until the edges are lightly golden and a toothpick in the center comes out with just a few moist crumbs. Cool completely in the pan on a rack.
• For the glaze, whisk the confectioners' sugar with the lemon juice and 1 teaspoon of the milk. Add the remaining milk as needed to reach a thick-but-pourable consistency.
• Spread the glaze over the cooled brownies and let it set for 20–30 minutes. Lift out using the parchment and cut into 9 squares.