Heat the milk slightly (to lukewarm). In a large bowl, combine the brown sugar, yeast, milk, rolled oats, and enough flour (1-½ to 2 cups) to make a sponge the consistency of bubbly pancake batter. Set it aside for a while (about 20 minutes) to allow the yeast to work. Then add the eggs, salt, and ½ cup of the softened butter. Stir well. Add the rest of the flour and knead it into a soft dough. (Add the last cup while kneading, if you wish.)
Let the dough rise in a large buttered bowl covered with a damp towel, until it has doubled in size, at least 1 hour. Divide the dough into two pieces, and roll them out into two flat rectangles about ½ inch thick, 12 to 15 inches long, and 10 inches wide. Spread the remaining ½ cup softened butter onto the rectangles. Distribute walnuts and raisins (if using) evenly, and sprinkle with most of the cinnamon sugar. Roll up the dough the long way and pinch it shut. Slice each roll of dough into 12 pinwheel pieces, brush with egg yolk, and sprinkle with remaining cinnamon sugar.
Fit the rolls into one or two buttered round or rectangular pans (use whatever you have on hand; the rolls can be squeezed in or given lots of room), and let them rise again until they’re puffy and doubled, about 45 minutes to an hour. Bake in a preheated 350 degrees F oven for about 25 to 30 minutes, or until they are cooked through and lightly browned. While they’re still warm, ice with frosting.