To make the dough: 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 large 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 hours, until it's nearly doubled in bulk.
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 log.
Braid the logs together, pinching them at both ends and tucking the pinched ends under.
Place the braid on a lightly greased or parchment-lined baking sheet, cover it, and allow it to rise for 45 minutes.
Toward the end of the rise, preheat the oven to 375°F.
To make the glaze: Whisk the egg yolk with the water, and brush the braid with this glaze.
To bake the braid: Bake the bread for 30 to 35 minutes, or until it's golden brown and shiny.
Remove bread from the oven and let it cool on the pan for 10 minutes, then transfer it to a wire rack to cool completely.