Casatiello Sponge

Stir in the yeast and 1 teaspoon of sugar into the water in a mixing bowl and let stand until foamy, about 10 minutes. Add the egg yolks and remaining sugar and stir until smooth. Add the remaining flour and the salt and stir until a soft dough is formed. Knead gently on a floured surface for 3 to 4 minutes. Place in a bowl, cover with plastic wrap and let rise until double in size, about 1 hour.