To make the dough: Dissolve yeast and 1 teaspoon sugar in 1/2 cup warm water. Mix warm milk, remaining sugar, and salt in a large bowl; mix in egg yolks, melted butter, yeast mixture, and 2 cups flour. Stir in remaining flour. Knead dough on lightly floured surface until smooth, 5-10 minutes. Place dough in a buttered bowl, cover, and let it rise in a warm place until it has doubled in bulk, about 1 hour.

To prepare the filling: Heat half-and-half to boiling in a large saucepan; stir in walnuts, sugar, salt, and vanilla. Brown the breadcrumbs in 4 tablespoons butter; stir the breadcrumb mixture into the walnut mixture; let the resulting mixture cool. Beat the egg whites in a clean bowl with an electric mixer until stiff peaks form; gently fold the beaten egg whites into the filling.

To form potica: Butter 3 to 4 loaf pans. Cover a large table with a clean, smooth tablecloth; sprinkle the cloth with flour. Roll dough out on the cloth into a 3-by-4-foot rectangle that is about 1/4-inch thick. Spread the filling over the surface of the dough. Starting on a short side, roll the dough up into a tight coil. Cut this into 3 to 4 loaves. Place one loaf in each of the buttered pans and let them rise in a warm place for about 1 hour.

To bake potica: Heat oven to 350 degrees. Bake loaves 40-50 minutes, brushing with melted butter during final 15 minutes of baking. Let loaves cool in the pans 10 minutes, then run a knife around the edges and turn them out onto cooling racks. Serve warm or at room temperature, sliced thick.