Preheat oven to 350 degrees.
Line a 9 inch square pan with parchment paper, allowing extra to spill over the sides.
In a stand mixer, beat butter until fluffy.
In a separate small bowl, combine rice flour, tapioca starch, brown sugar, potato starch and xanthan gum.
Add dry ingredients with the butter and mix until crumbly.
Press into baking pan.
Bake for 20 minutes.
Remove and cool.