Stir flour with yeast, 1/4 tsp salt and sugar in the bowl of an electric stand mixer with paddle attachment, until combined. Add 3/4 cup lukewarm water and beat on medium speed until mixed, about 5 min. Remove dough from paddle and bowl. Lightly spray bowl with oil and return dough. Cover with a damp kitchen towel (not terry cloth) and let rise at room temperature until doubled in size, about 1 hour.
Divide dough into 6 equal portions on a lightly floured counter. Cover with kitchen towel and let sit for 20 more min.
Preheat oven to 475F. Lightly spray a baking sheet with oil. Roll each portion of dough into a 10-in. baguette-style stick. Lay crosswise on prepared sheet. Stir baking soda with 1/4 cup hot water. Generously brush dough with mixture, then sprinkle with remaining 1/2 tsp salt.
Bake in centre of oven until golden, 8 to 10 min. Remove from oven and immediately brush with butter.