Chocolate Sauce:
Combine cornstarch and cocoa.
Whisk into cold water in a saucepan until there are no dry bits.
Cook, stirring frequently, until mixture comes to a boil.
Stir 1 minute until mixture thickens.
Remove from heat, stir in sweetener and vanilla.
Allow to cool.
Store in clean jar in refrigerator up to 6 weeks.

Shake:
Combine shake ingredients in a blender until smooth.
Serve immediately.