Preheat oven to 350. Grease bottom and sides of
9-inch square backing pan.
Melt butterscotch and butter in a 3-quart heavy
sauce pan over medium heat. Stirring constantly.
Remove from heat, cool to lukewarm.
Add flour, brown sugar, baking podwer, salt,
vanilla, and egg to the butterscotch mixture.
Fold in marshmallows and chocolate to butterscotch
batter just until combined.
Spread in prepared pan. Bake at 350 for 20 to 25
minutes. DO NOT OVERBAKE. Center wil be jiggly but
becomes firm when cooled.