Dissolve the yeast in the water.
Scald the milk, and pour it into a large mixing bowl. Add the butter and sugar, and stir until the butter is melted. Beat in the eggs and yolk. When the mixture has cooled to lukewarm, add the dissolved yeast. Mix in the salt and 4 cups of the flour, 2 cups at a time.
Turn the dough out on a lightly floured surface and knead, adding a little extra flour if needed if the dough feels too sticky. Continue to knead until the dough is soft, smooth, and elastic, 2 minutes.
Put the dough in a greased bowl, turning it to coat the surface. Cover the bowl with plastic wrap, place it in a warm spot, and leave it until the dough has doubled in bulk, 1 1/4 hours.
When the dough has risen, it may be punched down and completed, or it may be covered and set in the refrigerator for several hours or overnight.
When you are ready to make the kuchen, remove the dough from the refrigerator and turn it out onto a lightly floured surface. Knead it for a second or two, then
ill the kuchen with the Streusel mixture, folding to swirl in all of the mixture.
Place dough in a 13"X9" Pan with is greased with butter. Make Topping Thick with brown sugar. Spread topping over bread just before putting in oven. Bake at 400º for 25 minutes