1. In a large bowl, combine 1-1/3 cups flour, sugar, undissolved yeast, and salt. Heat water and butter until very warm (120o to 130oF). Gradually add to flour mixture. Beat 2 minutes at medium speed of electric mixer, scraping bowl occasionally. Add eggs 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.
2. Remove dough from refrigerator; roll to 20 x 10-inch rectangle. Brush with melted butter; sprinkle with Cinnamon Filling. Beginning from long side, roll up tightly as for jelly roll. Pinch seam and ends to seal; cut into 18 pieces. Arrange rolls, cut sides up, on greased large baking sheet to form a heart.* Cover; let rise in warm, draft-free place until doubled in size, about 45 minutes to 1 hour.
3. Bake at 375° F for 18 to 20 minutes. Remove from sheet; cool on wire rack. Drizzle with Powdered Sugar Glaze.
4. Cinnamon Filling: In a small bowl, combine 1/2 cup sugar and 2 teaspoons ground cinnamon. Stir to blend.
5. Powdered Sugar Glaze: In a small bowl, combine 1 cup powdered sugar, sifted; 1 to 2 tablespoons milk, and 1 tablespoon melted butter or margarine. Stir until smooth.

* A 12-inch heart-shaped pan may be used, if available.