Dissolve the yeast in a cup with a little water. Sift the flour and form it into a well on a work surface. Place the sugar, salt, oil and water in the center. Then start incorporating the flour until it has absorbed all the liquid. At this point, add the yeast and knead carefully. Continue working the dough until smooth. Cover, and let rest for 40 minutes in a warm place. Then, roll out the dough using a rolling pin until it is soft and you can cut out 4 pizzas about 9 1/2 in across. Create and edge by pressing your thumbs along the dough. Place the pizza dough on a floured pizza peel, one at a time. Place the pieces of tomato, slices of prosciutto, cubes of mozzarella, a drizzle of oil and a little salt on each pizza. Slide pizza on to a pizza stone in a hot oven (around 400 °F) for 20 minutes.