Cream together butter, white sugar and brown sugar.
Beat in egg and vanilla extract.
Stir in flour, baking soda and salt with a plastic or wooden spoon.
Stir in chocolate chips and marshmallows.
Drop by rounded tablespoonfuls about 2 inches apart on an un-greased cookie sheet. Bake for 8-10 minutes, centers will be soft and tops will have a "toasted" look. Cool slightly, remove from cookie sheet and enjoy.