Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with foil or parchment paper. Set aside.
In a large bowl, whisk together the butter and sugar until combined. Add the egg, egg yolk, lemon juice, and zest, and whisk until smooth and combined.
Add the flour and salt, and whisk until completely combined.
Spread the batter evenly into the prepared pan and bake for 30 - 35 minutes, or until the edges are browned and the centre is no longer jiggly - just set.
Allow to cool completely in the pan on a wire rack before glazing.
For the Glaze
In a small bowl, whisk together the icing sugar and lemon juice until completely smooth and combined. If the glaze is too runny, add more icing sugar.
Pour over the cooled brownies and spread out evenly. Sprinkle on some lemon zest, or top with lemon slices.
Allow the glaze to set at room temperature or in the fridge before slicing and serving.
Video
Notes
Brownies can be stored in an airtight container in the fridge for up to 1 week. They also freeze well for up to 3 months. Thaw overnight in the fridge before serving.