In stand mixer fitted with a paddle attachment, add 1 1/2 cups flour, yeast, sugar and salt; mix well. In saucepan or microwave-safe dish, heat water, sour cream and butter to 120-130°F (butter does not need to melt). Add to flour mixture; blend on low speed for 30 seconds. Add egg; beat on medium speed for 3 minutes.
Switch to dough hook attachment. Gradually stir in enough remaining flour to make a soft dough. Knead until smooth and elastic, about 5 to 7 minutes.(NOTE: For hand mixing: following above steps, mix ingredients in large mixing bowl using a wooden spoon or dough whisk. Knead on floured surface until smooth and elastic.) Place dough in lightly oiled bowl and turn to grease top. Cover; let rise until indentation remains after poking dough with finger down to second knuckle, about 1 hour.
Make the filling: In a small bowl combine filling ingredients and blend until smooth.
Turn dough onto lightly floured surface; punch down to remove air bubbles. Divide into 2 equal parts. On lightly floured surface, roll each half to 15x12-inch rectangle. Spread with half of filling. Starting with longer side, roll dough tightly. Pinch edges to seal. Place roll seam-side down on greased cookie sheet. Loosely coil foil to form a snail shape. Cover; let rise until indentation remains after lightly touching side of loaf.
Preheat oven to 350°F.
Bake for 20 to 25 minutes until golden brown; cool on rack. If desired, sift powdered sugar over top before serving.