1Mix sour cream, vanilla, and egg yolks together.Cut butter into flour until well combined, it should be crumbly.Add wet ingredients into flour mixture. A dough hook on a stand mixer is perfect for this step. Mix only until a ball forms, it will be sticky. Form a log shape, wrap and refrigerate overnight.
2When ready to bake, beat egg whites with cream of tartar and sugar until stiff peaks form.Preheat oven to 400 degrees. Slice dough into 15 to 20 pieces.
3Dust surface with powdered sugar and roll out dough portion into a very thin circle. Spread with about 1/4 cup of the beaten egg whites. Sprinkle with cinnamon and powdered sugar. Cut into eight wedges
4Roll each wedge, wide side in like a crescent roll. Roll them loosely. If you roll them too tight they get cakey, you want a flaky result. Place them on a cookie sheet, they can be close (about 1/2 inch apart) as they don't spread much, but not touching.
5Bake at 400 degrees for about 10 minutes or until golden brown. Remove immediately and cool on a wire rack. If you leave them on the cookie sheet too long, you'll need a hammer and chisel to get them off. Cool completely and dust with more powdered sugar.