In a large bowl, whisk together 1 1/2 cups of the flour with the yeast, sugar, salt and baking soda; mix well. Heat the yogurt, water and butter to 120-130°, but do not simmer. Add yogurt mixture to the flour mixture and blend well, then beat with an electric mixer at medium speed for 4 minutes. Gradually stir in enough of the remaining flour to make a stiff dough. Knead on a floured surface until the dough is smooth and elastic.
Place the dough in a greased bowl and turn the dough over to grease the top. Cover and let rise in a warm place until doubled in bulk (about 20 to 30 minutes). Punch down the dough, divide it into 24 pieces and form the pieces into balls. Place the balls on a greased baking sheet, cover and let rise in a warm place for about 20 minutes or until almost doubled in bulk.
Bake in a preheated 400° oven for 20 minutes or until nicely browned. Remove from pan and brush with butter.