Dissolve yeast in the warm water. Melt butter.
Sift flour with 3/4 C of the sugar and the salt into a large bowl.
Beat egg yolks until thick and light; blend in the sour cream and melted butter; stir in the dissolved yeast.
Gradually stir the egg yolk mixture into the dry ingredients, mixing to make a soft, smooth dough. Turn out on a lightly floured board and knead for 5 minutes, or uintil the dough is smooth. Turn into a bowl, cover, and let rise until almost doubled in bulk.
Prepare cheese filling. Cream the cheese until light and blend in the remaining 1/2 C sugar. Add the whole eggs, one at a time, and beat until smooth. Blend in vanilla.
Punch down dough and knead a few minutes. Lightly flour a board; roll out dough into a large circle, about 18 inches in diameter, and lay it over a greased 3-quart ring mold (you may use two 9" ring molds. Roll 1/2 dough 13" thick for each 9" ring mold). Fit the dough down into the bottom and the sides of the ring mold, being careful not to poke holes in it, and let it hang over the outside. Pour in the cheese filling (1/2 cheese fill for each 9" ring mold).
Lift outside edges of the dougn, and lap it over the filling, sealing to the inside ring of dough. Cut a cross in the dough which covers the center of the ring mold and fold each triangle formed back over the ring, completely encasing the filling.
Let rise until doubled in bulk or until the dough comes to the top of the pan.
Bake in a moderate oven (350 degrees) for 40 minutes, or until golden brown and a toothpick inserted comes out clean. (For 9" ring mold, bake for 30 minutes.) Let cool for 10 minutes, then turn out with top side down onto a rack.
When bread ring is cool, heat jam until it runs easily; carefully spoon jam over the ring. When set, dust lightly with powdered sugar by shaking about 2 tbsp sugar through a sieve. To serve, slice the ring in 1" wedges.