In a large bowl of electric mixer, sprinkle yeast over the warm water (105° to 115°F). Add 1 teaspoon of the agave. Let stand until soft (about 5 minutes). Stir in milk (105° to 115°F), remaining agave, salt, and the butter. Add 3 cups flour. Blend, then beat at medium speed until smooth and elastic (about 5 minutes). Stir in about 2 cups more flour to make a soft dough. Turn dough out onto a board or pastry cloth coated with some of the remaining 1/2 to 1 cup flour. Knead until dough is smooth and satiny and small bubbles form just under surface (10 to 15 minutes), adding just enough flour to prevent dough from being sticky. Cover ball of kneaded dough with plastic wrap and a kitchen towel; let rest for 30 minutes. Punch dough down and divide into two equal portions. Shape each into a loaf. Place shaped loaves in greased 4 1/2- x 8 1/2-inch loaf pans. Brush lightly with oil. Cover with plastic wrap and refrigerate for at least 3 hours (up to 24 hours). During this time, dough should nearly double in bulk. Remove from refrigerator and let stand at room temperature for 1 hour.

Preheat oven to 375°F. Bake until loaves are well browned and sound hollow when tapped (35 to 40 minutes). Remove loaves from pans and cool on wire racks. Rub 1/2 teaspoon of the soft butter (if desired) over top crust of each loaf while warm.