Warm the sour cream to lukewarm in a microwave, or over very low heat while stirring constantly. Add the butter and stir until melted.
In another bowl, whisk together the flour, yeast and salt. Add the dry ingredients to the sour cream mixture, then stir in 1 egg and 1 egg white. Stir until the mixture comes together to form a shaggy mass, then set it aside for 15 minutes.
Knead the dough -- by hand, electric mixer, food processor or bread machine -- until it's smooth and elastic, adding a small amount of water or flour, if needed. Place it in a greased bowl, cover the bowl, and set the dough aside to rise for 1 1/2 to 2 ho
Turn the dough out onto a lightly greased surface and knead it gently a few times, to expel the excess carbon dioxide. Divide it into three equal pieces, and roll each piece into an 18-inch log. Braid the logs together, pinching them at both ends and tuck
Whisk the egg yolk with the water, and brush the braid with this glaze. Bake the bread in a preheated 375°F oven for 30 to 35 minutes, or until it's golden brown and shiny. Remove it from the oven and let it cool on the pan for 10 minutes, then transfer i