Preheat your oven to 350ºF (180ºC) and line a 9-inch square baking pan with parchment paper.

Combine sugar and lemon zest in a large bowl, rubbing the zest into the sugar.

Mix in the melted butter, eggs, egg yolks, and vanilla; whisk until fully combined.

Stir in the lemon juice and whisk well.

In a separate bowl, whisk together the gluten-free flour blend, xanthan gum, and salt.

Combine the dry ingredients with the wet mixture, whisking until smooth.

Transfer the batter into the prepared baking pan, spreading it evenly.

Bake for 20-22 minutes, or until the top is golden brown and a toothpick comes out mostly clean.

Cool completely in the baking pan before removing.

Prepare the icing by whisking together the icing ingredients until smooth.

Pour the icing over the cooled brownies and spread evenly.

Set the icing at room temperature before slicing and serving.