Melt the butter in a large skillet. Stir in the marshmallows, milk, and pumpkin. Stir frequently until melted and smooth. Remove from heat; stir in vanilla and pumpkin pie spice. Cool completely, about 30 minutes.
Combine 1/3 cup confectioners' sugar and 1 cup heavy cream in a large bowl. Use an electric mixer to beat until stiff peaks form. Fold whipped cream into cooled pumpkin mixture. Pour mousse into ramekins or chocolate shells. Cover and refrigerate until firm, about 2 hours.