This recipe has truly been kept a "Secret" for 30 years but now is the time to release it. It was given to me by an elderly lady who had been given it by an even more elderly Swedish lady. The proviso: "After I'm 'gone', you may give out the recipe." The same proviso was given to me...so, here it is.
Preheat oven to 350 degrees. In a mixer fitted with a paddle attachment, cream the butter and sugar. Add the yolks and vanilla, mixing well. Add the flour and combine thoroughly.
Use mounded teaspoonfuls and make balls of dough with your hands. Place on an ungreased cookie sheet, then flatten the dough with the bottom of a patterned glass dipped in colored sugar (don't mix the red and green!).
Bake for about 10 minutes (watch carefully as they burn easily), until the cookies are lightly golden just around the edges. Let the cookies rest on the baking sheets for a minute or two and then gently transfer to baking racks to cool -- they're fragile.