Combine the flour, salt and sugar in the bowl of a food processor and process a few seconds to mix. Quarter the stick butter lengthwise and cut into 8 or 9 pieces. Scatter the cubes of butter evenly over the flour. Pulse the machine until the butter pieces are about the size of lentils.
Place the lightly beaten egg in a 1/4 cup measure and fill to the top with ice water. Put half the liquid in the bottom of a bowl. Turn the flour/butter mixture onto it and pour the remaining liquid evenly over the top. Using a large rubber spatula, fold the mixture until it's evenly moistened. Squeeze it; it should hold together. If not, sprinkle in a little more water and gently mix.
Turn the mixture out on the table and quickly frisage: (Using the palm of your hand, push sections of the dough away from you against the table. Gather the dough together and frisage any portions that were missed.) Form the mass into whatever shape you will be rolling out (i.e., disk for round, cylinder for rectangle), and wrap in plastic wrap. Chill for at least 30 minutes or up to 3 days. Dough can be frozen for up to 3 months.