Adjust the oven rack to the middle position, pre-heat the oven to 350ºF (180ºC) and line a 9-inch (23cm) square baking pan with parchment/baking paper.
Tip: When lining the baking pan, I recommend leaving some parchment/baking paper overhang, as that will help with removing the lemon brownies from the pan later on.
Add the sugar and lemon zest to a large bowl, and use your fingertips to rub the zest into the sugar.
Tip: This helps to release more essential oils from the zest, and it will make your lemon brownies even more lemony and aromatic.
Add the melted butter, eggs, egg yolks and vanilla, and whisk until well combined and smooth.
Tip: Don't aerate the mixture – you don't want to whisk it until it's pale and fluffy, but just until all the ingredients are combined. Introducing too much air into the batter can make the lemon brownies cakey rather than fudgy.
Add the lemon juice and whisk well to combine.
In a separate bowl, whisk together the gluten free flour blend, xanthan gum and salt.
Add the dry ingredients to the wet and whisk well until you get a smooth, fairly runny batter with no flour clumps.
Transfer the batter into the lined baking pan and smooth out the top.
Bake at 350ºF (180ºC) for 20-22 minutes or until they’re golden on top, an inserted toothpick comes out mostly clean and they’re still soft to the touch when you gently press down on them with a finger.
Tip: It's better to under-bake rather than over-bake these, as you want them to be super fudgy.
Allow the lemon brownies to cool completely to room temperature in the baking pan before you spread on the lemon cream cheese icing.
Lemon cream cheese icing:
In a bowl, combine all the lemon cream cheese icing ingredients. Whisk well until you get a smooth, runny icing with no clumps.
Pour the icing over the (completely cooled) lemon brownies and use a small offset spatula or the back of a spoon to smooth it out into an even layer.
Allow the icing to set at room temperature (if you have the patience) before removing the lemon brownies from the pan, slicing and serving.
The addition of cream cheese (with its extra moisture and fat) means that the icing will stay “wet” and sticky to the touch for longer – depending on the ambient humidity, it can take 1-2 hours for it to be dry to the touch.
If you want an icing that sets and dries out quicker, make a simple lemon icing instead, with 120g (1 cup) of powdered/icing sugar, 6-7 teaspoons of lemon juice, lemon zest and some vanilla (and no cream cheese).