Preparation Steps

• Heat the oven to 325°F. Line an 8x8-inch light-colored metal pan with parchment so the sides overhang; lightly grease the exposed sides of the pan.
• Bloom the zest: Stir the lemon zest into the warm melted butter and let it stand 5 minutes to infuse, then whisk to distribute evenly.
• In a large bowl, whisk the sugar and eggs together until slightly thickened and lighter in color, 45–60 seconds by hand.
• Slowly stream the lemony butter into the egg mixture while whisking. Whisk in the lemon juice, lemon extract, vanilla, and salt until smooth.
• In a separate bowl, whisk together the gluten-free flour blend and cornstarch. Add the dry mix to the wet ingredients and fold with a spatula just until no dry streaks remain; the batter will be thick.
• Rest the batter 10 minutes to hydrate the gluten-free flours. Scrape into the prepared pan, smooth the top, and tap the pan once on the counter to level.
• Bake for 26–30 minutes, until the edges are lightly golden and a toothpick inserted near the center comes out with moist crumbs (not wet batter).
• Cool the pan on a rack for 15 minutes, then use the parchment to lift the slab out. Cool until just barely warm, about 45 minutes more.
• Make the glaze: In a small bowl whisk the powdered sugar with lemon juice until smooth and thick but pourable. If needed, whisk an extra 10–15 seconds to loosen—avoid adding extra liquid so the glaze sets opaque.
• Glaze while warm: Spread the glaze over the barely warm brownies; it will settle into a glossy shell. Let stand 20–30 minutes to set. Chill 10 minutes for the cleanest slices, then cut into 16 squares with a hot, wiped knife.