Grease a 9" square pan with nonstick spray or butter.
Combine half & half and sugar in a large heavy saucepan. Bring to a boil; reduce heat to medium. Stir constantly and continue a rolling boil for 5 minutes.
Remove pan from heat and add marshmallows , chocolate chips and vanilla extract. Stir until marshmallows are melted.
Quickly pour into a greased pan. Cool; cut into 3" x 1-1/2" pieces, making 18 pieces.