Pour the half-and-half into a medium saucepan and bring to a simmer over medium heat. Whisk in the sugar substitute, cocoa powder, tapioca and salt. Cook and stir until soft, approximately 2 minutes. Then whisk in the margarine, vanilla extract and butter-flavor extract.
Pour the mixture into a large serving bowl. Cover with foil and refrigerate for at least 3 hours, or up to 2 days.