Combine all ingredients except the cornstarch and water in a medium saucepan.
Whisk everything together and bring to a boil over medium-high heat.Be careful not to turn the heat up too high as it can burn the sugar in the soda.
Combine cornstarch and water to make a slurry. Add the slurry to the saucepan.
Reduce heat to low and simmer, uncovered, for 20-30 minutes, or until sauce is thickened.20-30 minutes may seem like a long time, but you need time for the water in the Dr. Pepper to evaporate and cook off.