In large bowl, cream butter and sugar until light and fluffy. Beat in yolk, one at a time, until pale and fluffy. In small bowl, soften yeast in water,beat in egg mix salt,peel, milk and yeast. Slowly beat in 5 1/2 cups of flour until a soft dough forms. Turn out on lightly floured work surface, knead 8-10 minutes, until smooth and elastic. Knead in raisins. Place in greased bowl. Grease top. Let rise 1 hour, or until doubled in bulk. Punch down, divide in half. Place in 2 greased and floured 8 inch fluted pans, let rise until double in bulk(covered) Bake in 350 F oven 25-30 minutes, or until well browned. Invert on rack. Sprinkle with confectioner's sugar. Tastes better eaten within a couple of days. Two Babkas, 10 servings each.