Step 1Dissolve the yeast in the milk and allow to stand until tiny bubbles appear on the surface, 3 to 5 minutes.
Step 2Cream 1 cup of butter until fluffy, then add the sugar, then the eggs, one at a time, stirring well after each addition. Add the salt and lemon zest. Stir in the flour alternately with the yeast mixture and mix well. (If the dough is too soft to knead, add more flour as needed.)
Step 3Knead the dough on a floured board until it's smooth and elastic (or in a mixer fitted with a dough hook attachment), 8 to 10 minutes.
Step 4Place the dough in a greased bowl, cover it with a damp towel, and let it rise in a warm place until doubled in bulk, 45 minutes.
Step 5Spread the dough 1/2 inch thick in a shallow buttered 18x12-inch (half sheet) pan. Cover and let it rise again until doubled in bulk, 45 minutes.
Step 6Heat the oven to 350 degrees.
Step 7Melt the remaining 2 tablespoons of butter and spread it over the dough. Sprinkle the dough with more sugar and the cinnamon and chopped nuts, if desired.
Step 8Bake the kuchen until golden, 15 to 20 minutes.