To make the caramel filling, heat the sugar in a medium saucepan with a heavy bottom, stirring constantly. The sugar will lump and then melt to a deep golden brown. Watch carefully—sugar burns easily. Add the bacon drippings and the butter, if using, and stir vigorously. Remove from the heat and drizzle in the cream carefully (it will steam and bubble), beating rapidly to mix. When the mixture is smooth and the bubbling has calmed, set it aside to cool for at least 15 minutes.