In a large bowl, or food processor, measure out the brown rice flour, tapioca starch, potato starch, salt, and sugar.
Using a knife, cut the butter in small-ish pieces into the flour.
If you're using a food processor, pulse about 20-30 seconds until the butter is coarsely chopped into pea-sized pieces. Alternatively, without a food processor, you can use your fingers to break up the butter into small, pea-sized pieces. The end result should be course and crumbly, without any large pieces of butter.
Use a spoon to mix in the egg and cold water. Once the mixture begins to form together, use your hands to help it come together into a ball of dough, with no dry spots. The dough should not be dry or crumbly at all, and should resemble the texture of play-doh.
Divide the dough into two even pieces and form a ball with each. Then press and flatten each ball into a round, disc shape.
Wrap each ball in plastic wrap or a recycled plastic bag and place in the fridge for a minimum of two hours before use.