Make the starter: Mix the starter ingredients in a medium bowl until smooth. Cover with plastic wrap, and leave the starter out at room temperature overnight.
Prepare the dough: In the bowl, combine the starter, yeast, water, flour, and salt. Run the mixer on low until the dough is smooth and soft with a somewhat rough surface, about 5 minutes.
Place the dough in a large, lightly buttered bowl, and cover it with lightly buttered plastic wrap. Let the dough rise for 3 hours.
Turn the dough out onto a lightly buttered counter top. Divide the dough into three pieces. Cover the pieces with lightly buttered plastic wrap, and let them rest for 15 minutes.
Meanwhile, lay a dish towel in a 12-by-17-inch sheet pan and rub it with flour. You'll put your shaped baguettes on the towel and coddle them in the folds of the towel. Line another 12-by-17-inch sheet pan with parchment.
Shape the dough: Working with one piece at a time, flatten the dough, fold it in half lengthwise, and seal the edges with the heel of your hand. Flatten the dough again, fold it in half lengthwise, and seal the edges with the heel of your hand. Then with the seam side down, gently roll the dough into a 15-inch log.
Transfer the logs to the sheet pan with the floured towel. Cover them with buttered plastic wrap, and let them rise until they become puffy, 60 minutes.
Preheat oven to 450 degrees. Roll or carefully lift the baguettes onto the sheet pan lined with parchment. Make sure to turn them floured side down. Fill a small bowl with water, dip your fingertips in the water, and flick the baguettes with the water. Also, fill a pan with ice cubes and place it on the bottom rack in the oven.
Using a very sharp knife held at a 45-degree angle, make three 8 vertical slashes in each baguette.
Transfer the sheet pan to the oven, and bake the baguettes for 25 minutes.Turn off the oven and leave it open a crack, and let the baguettes cool in the oven for a very crispy crust.