Preheat oven to 375degrees. In a large bowl, combine butter, shortening, sugars and mix well. Add eggs and beat until all are blended. Add vanilla and sourdough starter. Beat in flour, soda, salt and mix well. Stir in walnuts and chocolate chips. Drop by tablespoons onto greased cookie sheet. Bake 10 to 12 minutes. This makes 70 cookies and can be cut in half for smaller batches.