1. Sift the flour. Dissolve our sourdough starter in the water as thoroughly as possible. Add the flour, then the salt. Be careful not to incorporate any air into the mixture. Knead for about ten minutes until you have a homogeneous ball of dough. It shouldn't stick to your hands or the sides of your bowl.
2. Cover with plastic wrap and let the dough rest for about an hour. During warmer months, do not cover it with a blanket. Place the dough at room temperature. Afterward, weigh it and divide it into two equal parts. Form two balls, being careful not to trap any air inside.
3. Using a rolling pin, roll out two uniform discs. The diameter of our disc should be slightly smaller than the diameter of our prosforo seal. If you notice any bubbles on the surface of our disc, prick them with a pin.
4. Next, take one disc, which will be the bottom part of the prosforo, brush it with water, and place the other disc on top. Seal the prosforo by pressing firmly. Prick it in several places with a toothpick and let it rest for another hour. This is to allow it to 'settle' or 'rise slightly'. Don't leave it for too long, as we don't want it to rise too much and lose our seal's impression, or for bubbles to form inside the prosforo.
5. Bake in a preheated oven at 170°C for one hour, depending on your oven. We don't want it to get too much color or become intensely browned on the surface. We want it to bake thoroughly from the inside. If it browns too quickly on top, place a piece of aluminum foil over it to protect it.
