Feed sourdough starter and let sit at room temperature for 8 to 12 hours.
Dissolve yeast in warm water in a large bowl. Add 4 cups all-purpose flour, sourdough starter, sugar, and salt. Cover and let rise until double in size, about 1 hour.
Mix remaining 1 cup of flour with baking soda in another bowl. Stir mixture into dough slowly, adding just enough to make dough stiff. Knead dough, adding flour mixture as needed, until smooth and shiny. Shape into two baguettes and place on a baking sheet sprinkled with cornmeal. Let dough rise again until double in size, 1 to 1 1/2 hours.
Preheat the oven to 400 degrees F (200 degrees C).
Brush tops of baguettes with cold water and make several gentle slashes along the tops of the loaves.
Place a small, oven-safe, pan of water on the bottom rack of the oven. Bake baguettes in the preheated oven until golden brown, about 25 minutes. Remove from the oven and brush each with melted butter. Continue baking until crisp, 3 to 5 minutes more.
