Cream butter and sugar.
Beat in egg, sourdough starter, cream or milk, and vanilla.
Stir in flour, baking powder and salt.
If you would like to ferment your dough, cover with plastic wrap and let it sit at room temperature for about 2-4 hours. Then place in the fridge over night to ferment a little more. If you are worried about the eggs and dairy in the dough, you can wrap two discs of dough in plastic wrap and place in fridge right away for up to three days.
If you are not fermenting, place dough in the fridge to chill. This is optional, however I find that when I chill the dough my shapes turn out way better.
Preheat oven to 400 degrees F.
Roll out dough to about 1/8 to 1/4 inch depending on your preference on a lightly floured surface. Thicker cookies will require longer baking time. Use desired cookie cutters to cut out your shapes.
Line baking sheets with parchment paper and place cookies about one inch apart. Sprinkle with sugar if desired.
Bake anywhere from 5 to 12 minutes depending on size of cookie. The cookie cutters in my pictures are about four inches and the cookies were a little smaller than 1/4 inch. I baked them for 11 minutes in my oven. The cookies should be slightly browned on the edges. I was able to get 22 cookies out of the dough.
Let cool before frosting if desired.