FOR THE SPONGE: Dissolve the sugar in the lukewarm water, sprinkle the yeast over it, and let it stand until softened. Combine with the lukewarm milk and 3/4 cup of flour. Beat well, cover and let the sponge rise in a warm place until light and bubbly.
FOR THE SWEET DOUGH: Cream the butter with the sugar. Beat the whole eggs and the egg yolks together along with the salt. Combine with the butter sugar mixture and beat thoroughly. Stir in the vanilla, lemon rind and sponge. Add the flour and knead in the bowl for about 10 minutes. This dough should be soft.
Cover and let it rise in a warm place until double in bulk. Punch down, knead a few times, and let it rise again. You can use this dough with any sweet filling such as cottage cheese, nut and poppy rolls, filled doughnuts filled with thick poppy seed, jam, or cooked prunes pitted and sweetened.