Sift the rice flour, fine cornmeal, potato flour, xanthan gum and salt into a bowl and mix well. Cut the butter into cubes and gently rub into the flour mixture. Stir in the sugar. Make a well in the centre and carefully add some of the egg and water mixture - just enough to bring the pastry together using a fork. Collect the pastry into a ball with your hands. This way you can judge more accurately whether you need a few more drops of liquid. It is tempting to add extra liquid at this stage but try not to, as it is very easy to make the pastry too wet. It is fine to have some liquid left over. Although rather damp pastry is easier to handle and roll out, the resulting crust can be tough and may well shrink out of shape in the oven.
On a very lightly rice-floured board, gently knead the dough with the heel of your hand for a few minutes to form a silky- smooth ball. Flatten slightly, wrap in cling film and refrigerate for about 30 minutes. This will make the pastry much less elastic and easier to roll.
When it has chilled enough, roll it out (between two sheets of parchment paper, if necessary, to stop it sticking) and use as required.