Line an 8x8x2-inch baking pan with wax paper; set aside.
In a 2-quart saucepan over low heat, melt butter and chocolate.
Remove from heat.
Stir in corn syrup, half-and-half, and vanilla until blended. Add
powdered sugar. Stir until blended and smooth. Stir in marshmallows or
nuts. Spread in prepared pan. Chill for 30 minutes.
Cut into squares. Cover and store in the refrigerator.