Combine 3/4 cup flour, sugar, undissolved yeast and salt in a large mixer bowl. Heat water, sour cream and butter until very warm (120° to 130°F). Gradually add to flour mixture. Beat 2 minutes at medium speed of electric mixer, scraping bowl occasionally. Add egg and 1 cup flour; beat 2 minutes at high speed. Stir in remaining flour to make a stiff batter. Cover tightly with plastic wrap; refrigerate 2 to 24 hours. For Cream Cheese Filling, combine cream cheese, sugar and vanilla in a medium bowl. Stir until smooth. Remove dough from refrigerator; roll to 16 x 8-inch rectangle. Spread evenly with Cream Cheese Filling. Beginning at long end, roll up tightly as for jelly roll. Pinch seam to seal. Form a large heart; pinch ends together. Place on greased baking sheet. With a sharp knife, cut 1/3 through heart at 1-inch intervals, alternating from side to side to resemble a heart. Cover; let rise in warm, draft-free place until doubled in size, about 1 hour.
Bake at 375°F for 20 to 25 minutes. Remove from sheet; cool on wire rack. Drizzle with Powdered Sugar Glaze. For Powdered Sugar Glaze, combine powdered sugar, milk and vanilla. Stir until smooth. Add 2 drops red food coloring if desired; stir until well blended.