Preheat oven to 400.
Cream together butter, sour cream, and sugar until smooth. Beat in one egg at a time, then vanilla.
Stir in salt, leavening, and flour, 1/2 c at a time.
Drop rounded spoonfuls of dough on baking sheets. Press dough with fingers until it is a disk about 1/4 inch thick.
Bake 7 minutes. With a very thin turner, removing cookies from baking sheets and cool on wire racks.
For frosing, cream sugar and vanilla into butter until smooth. Color frosting if desired. Spread over cooled cookies and top with sprinkles. Store airtight.