1. Mix yeast and warm water and permit it to rest for ten minutes and begin to bubble or "bloom".
2. Using a food processor, cut cold butter into the flour and salt. Process until it is the texture of sand. Whisk together the slightly beaten eggs, sour cream, yeast water and vanilla. Pour this mixture into the flour and process it until just combined.
3. Divide the dough in half and wrap each piece in plastc wrap and chill for at least two hours; the longer the better.
4. Mix the sugar and remaining vanilla; it should look like damp sand.
5. Dust your board with the vanilla sugar and then roll half of the dough into a rectangle. Dust the top of the dough again with the sugar mixture and then fold the top third down and the bottom third up. Roll out the dough and repeat two more times, paying attention to shape the dough into a rectangle.
6. Roll the dough, using the sugar to dust the board as needed to prevent sticking, into a large rectangle which is about 1/8-1/4" inch thick.
7. Place parchment paper on a cookie sheet. Cut the dough into rectangular pieces, roughly 1" x 4" and twist each piece before placing it on the cookie sheet, making sure that the ends lay flat.
8. Bake at 375 degrees for 10-15 minutes, until barely golden.

NOTE: Divide the dough into manageable portions so that you can roll it out and form the cookies before the dough begins to warm. The faster you can process the dough into cookies, the better. When warm, the dough is difficult to manipulate. And don't add too much sugar while "dusting".