Cream the fresh yeast with the sugar in a bowl, then whisk in the water. For dried yeast, dissolve the yeast in a bowl with the sugar, water and 2 tablespoons of the flour. Cover with a moist cloth and leave to stand in a warm place for 10 minutes until foamy.
Sift the flour into a large bowl and make a well in the centre. Add the yeast mixture, oil and salt to the well and mix with a round-bladed knife, then with your hands, until the dough comes together. Knead on a floured work surface for about 10 minutes until smooth and elastic. It should be quite soft, but if it is too soft to handle, add a little more flour. Put in an oiled bowl, cover with a moist cloth and leave to rise in a warm place for 1 hour, or until doubled in size.
Knock the dough back and roll out to a 30 cm (12 inch) circle, leaving the border slightly thicker. Slide on to a large floured baking sheet.
Spread the tomato purée over the pizza base. Arrange half the mozzarella on top. Scatter over the tomatoes, artichokes and garlic. Drape the prosciutto over the pizza. Scatter over the Parmesan and remaining mozzarella. Drizzle with oil and bake in a preheated oven, 240°C, (475°F), Gas Mark 9, for 15–20 minutes until golden. Serve garnished with basil leaves.