In a small bowl dissolve yeast in the warm water. Set aside.
Meanwhile, in a large mixing bowl place the 1/2 cup butter, the sugar, molasses, and salt. Add the boiling water; stir until butter is melted. Add rye flour. Beat with an electric mixer on low speed until combined, then on medium speed about 3 minutes or until smooth. Gradually beat in yeast mixture until combined. Using a wooden spoon, stir in as much of the all-purpose flour as you can.
Turn the dough out onto a lightly floured surface. Knead in enough of the remaining all-purpose flour to make a moderately stiff dough that is smooth and elastic (8 to 10 minutes total). Shape dough into a ball. Place in a greased bowl, turning once to grease surface of the dough. Cover; let rise in a warm place until double in size (1-1/4 to 1-1/2 hours).
Punch dough down. Turn dough out onto a lightly floured surface. Divide dough in half. Cover; let rest for 10 minutes. Shape each half into a loaf and place in 2 greased 8x4x2-inch or 9x5x3-inch loaf pans. Cover and let rise in a warm place until nearly double (40 to 50 minutes).
Bake in a 350 degree F oven for 35 to 45 minutes or until done. Remove from pans. Brush tops of warm loaves with a little softened butter. Cool on wire rack. Makes 2 loaves (24 servings).