Prep your pan and oven: Heat the oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment, leaving overhang on two sides for easy lifting. Lightly grease the parchment.

Mix the wet ingredients: In a medium bowl, whisk the melted butter and sugar until glossy and combined, about 30 seconds. Add eggs and the extra yolk. Whisk until the mixture looks thick and smooth. Stir in vanilla, lemon zest, and lemon juice.

Add dry ingredients: Sprinkle flour and salt over the wet mixture. Use a spatula to fold just until no dry spots remain. The batter will be thick and silky—don’t overmix.

Fill the pan: Spread the batter evenly in the prepared pan. Smooth the top with an offset spatula or the back of a spoon.

Bake: Bake for 22–28 minutes, until the edges are set and lightly golden and the center no longer jiggles. A toothpick inserted 2 inches from the edge should come out with a few moist crumbs.

Cool completely: Set the pan on a rack and let the brownies cool fully in the pan. This helps them set and stay fudgy.

Make the glaze: In a bowl, whisk powdered sugar, lemon zest (if using), a pinch of salt, and 2 tablespoons lemon juice. Add more juice, a few drops at a time, until the glaze is thick but pourable.

Glaze and set: Pour the glaze over the cooled brownies and spread to the edges. Let it set for at least 30 minutes, or chill for a cleaner cut.

Slice and serve: Lift out using the parchment, cut into 16 squares, and enjoy. Wipe the knife between cuts for neat edges.