Cook sour cream, sugar, butter and salt in a medium saucepan over low heat, stirring often, until butter melts. Set aside, and cool mixture to 100° to 110°.
Stir together yeast, 1/2 cup warm water, and 1 tablespoon sugar; let stand 5 minutes.
Beat sour cream mixture, yeast mixture, eggs, and 2 cups flour at medium speed with a heavy-duty electric stand mixer until smooth. Reduce speed to low, and gradually add remaining flour (4 to 4 1/2 cups) until a soft dough forms.
Turn dough out onto a lightly floured surface; knead until smooth and elastic (about 10 minutes). Place in a well-greased bowl, turning to grease top.
Cover and let rise in a warm place (85°), 1 hour or until dough is doubled in bulk.
Punch down dough, and divide in half. Roll each portion into a 22- x 12-inch rectangle. Beat 3/4 cup sugar, cream cheese, 1 egg, and vanilla at medium speed with an electric mixer until smooth. Spread cream cheese mixture evenly on each dough rectangle, leaving 1-inch borders.
Roll up each dough rectangle, jelly-roll fashion, starting at 1 long side. Place one dough roll, seam side down, on a lightly greased baking sheet. Bring ends of roll together to form an oval ring, moistening and pinching edges together to seal. Repeat with second dough roll.
Cover and let rise in a warm place (85°), 20 to 30 minutes or until doubled in bulk.
Bake at 375° for 14 to 16 minutes or until golden. Bake each cake separately and not at the same time. Slightly cool cakes on pans on wire racks (about 10 minutes).
Stir together powdered sugar, butter, lemon juice and vanilla extract. Stir in 4 tablespoons milk.
Drizzle Creamy Glaze evenly over cakes; sprinkle with colored sugars.
