Roll half of the dough out on floured surface. Cut into circles and transfer to an ungreased cookie sheet. If you don't have circle cutters, divide half of the dough in 6 balls. Press them with a plate, if you like them crunchy make them really thin, if you want them more bread like make them about 1/4 inch high. In a shallow container whisk up the egg with a splash of cold water or milk until pale yellow and perfectly mixed. Use a pastry brush and paint the champurradas with a light, even coat of egg wash, then sprinkle on top the sesame seeds.