In the bowl of an electric mixer fitted with the paddle attachment, combine 1 cup warm water and the yeast. Let stand until creamy, about 10 minutes. Add remaining 1/2 cup warm water and the sponge. Mix on low speed until combined, about 2 minutes.
In a medium bowl, combine flour and salt. Add to yeast mixture, and mix on low speed, 1 minute. Change attachment to dough hook, and mix on medium-low speed until dough is smooth and just sticks to your fingers when squeezed, about 8 minutes.
Lightly flour a work surface. Turn out dough, and knead 4 or 5 turns into a ball. Place dough, smooth side up, in a lightly oiled bowl, and cover with plastic wrap. Let rise in a warm place until dough has doubled in bulk and is slightly blistered and satiny, about 1 hour.
Punch dough down, and fold over 4 or 5 times. Place folded side facedown in bowl. Cover, and let rise again in a warm place until doubled in bulk and satiny, about 50 minutes. Divide dough in half, and wrap in plastic until ready to use.