Place chocolate and butter in a small saucepan. Melt over low heat. Add half-and-half and sugar. Cook and stir until slightly thickened, about 10 minutes. Remove from heat and freeze about 30 minutes or until cool.
Pour cooled chocolate mixture into crust. Place ice cream in a large chilled bowl and stir with a wooden spoon until smooth but not melted. Gently spread on top of chocolate mixture in crust. Cover and freeze overnight.
Uncover and let stand 10 to 15 minutes before serving. Swirl whipped cream over top and sprinkle with nuts.