Cream the butter until very light. Add sugar; cream butter and sugar until combined. Add egg yolks and sour cream and beat until combined.
Slowly add flour and baking powder . Mix on low speed until dough is uniform. Refrigerate until cold (about one or two hours).
Preheat oven to 325 degrees. Roll out some dough to about 3/8 of an inch thick. Refrigerate remaining dough. Cut cookies and place on ungreased cookie sheet. Brush with egg white or beaten whole egg. Bake 15 minutes until golden; watch carefully!
Notes: If you use jumbo eggs or are baking in a hot, humid kitchen, reduce yolks to 6.
A stand mixer works best. Most hand mixers don't have the power to work with this much cookie dough. If using a hand mixer, add the last bit of flour by hand.
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