Preheat oven to 375 degrees F. Cream softened butter with powdered sugar and granulated sugar. Add egg and vanilla and blend well. Mix flour with baking soda and gradually add to mix until dough becomes easy to handle by hand.
Roll dough into 1 inch balls and place on baking sheet. With handle of spoon or thumb make an indentation in each ball.
Bake for 10 minutes or until bottoms are golden brown. The tops of the cookies should still be very pale. Remove to cooling rack. Place approximately 1 tsp. of strawberry preserves in indentation in each cookie.