Preheat oven to 350F.
Butter a 9x13 inch baking pan.
In a large bowl, whisk together the flour, xanthan gum (if using), granulated sugar and salt until combined.
In a separate bowl, mix together the melted butter, sweetened condensed milk, eggs, vanilla, lemon juice and lemon zest.
Mix the wet ingredients into the dry ingredients until thoroughly combined.
Spread the batter into the baking pan and bake for 25-35 minutes (mine took 30) until a toothpick inserted in the middle comes out mostly clean, it still might be a little gooey. Let the bars cool before spreading the glaze on top.
To make the glaze, in a small bowl stir together the powdered sugar (confectioner's sugar), 3 tbsp fresh lemon juice, and 1 tsp lemon zest until it forms a smooth glaze.
When the bars have cooled, pour the glaze over the brownies and spread with a knife or spatula.
Allow the glaze to harden before cutting into squares.