Combine the sour cream, butter, salt and sugar in a small saucepan and heat until butter is melted. Let cool to about 105 degrees F.
Combine the yeast and water in a small bowl and let sit until it starts to foam.
Put the flour in a mixing bowl. Stir in the cooled sour cream mixture and the yeast to make a soft dough. Cover with plastic wrap and chill dough for at least 8 hours.
Punch down the dough (it probably will not appear to have risen much) and divide it into two parts.
Roll each part into a 12- x 8-inch rectangle, and spread each rectangle lengthwise with half the cream cheese filling, leaving a 1-inch border around edges. Carefully roll up, starting at a long side; press seam, and fold ends under to seal. Place, seam side down, onto a parchment lined or greased baking sheet. Cut 6 equally spaced "X"s across top of each loaf; cover (I recommend loosely if at all) and let rise in a warm place (85°), free from drafts, about 1 hour or until doubled in bulk.
Bake at 375° for about 20 minutes or until browned. Let cool, then drizzle with icing and serve. If you want the icing to hold its shape and look pretty, I recommend serving at room temperature. I was worried I'd have to warm the bread and deal with melted icing, but the bread actually tastes great at room temperature. If you want to serve it warm, you might consider putting the icing on at the last minute.
Icing: Mix all icing ingredients together until smooth. Put the icing in a freezer bag or pastry bag. When loaves are cool, snip off end of icing bag and drizzle it across the loaves. Let set.