Prep: 30 minutes; Stand: 2 1/2 hours; Bake: 35 minutes.
1. Combine milk and sugar in a small saucepan over high heat. Heat just until small bubbles appear around edges. Remove from heat. Stir in butter until melted; add salt. Let mixture cool to warm.
2. Dissolve yeast in about 1/4 cup warm water. Transfer milk mixture to bowl of a stand mixer fitted with paddle attachment. Add yeast mixture, and beat in 1 whole egg and 3 egg yolks. Add 3 cups flour and mace, and beat until smooth. Fit mixer with dough hook. Add remaining 2 to 3 cups flour, 1/2 cup at a time, mixing well after each addition, until smooth and elastic, not sticky. Add raisins, and mix well to combine.
3. Sprinkle a little flour over dough, place in a large bowl, cover, and let rise in a warm place about 1 1/2 hours or until doubled in bulk.
4. Place dough on a floured surface, and divide into 8 balls: 3 large (softball-size), 3 medium (tennis ball-size), and 2 small (slightly larger than golf ball-size). Knead each ball well. Roll large balls into 16-inch ropes, and braid together. Place on a baking pan. Repeat with medium balls, and place braid on top of first braid. Repeat again with small balls; place braid on top of second braid. (You may need to secure ropes with toothpicks to avoid slippage during rising and baking.) Cover and let rise 1 hour.
5. Preheat oven to 325°. Brush houska with melted butter, if desired. Bake at 325° for 35 to 40 minutes or until a wooden pick inserted in center comes out clean.