Preheat the oven to 350º.
In a medium bowl, combine all of the ingredients for the topping. Use your hands to mix the butter well. Refrigerate while you prepare the filling.
Chop rhubarb into small pieces, about the size of your pinky nail.
In a medium bowl, combine rhubarb, corn starch, rum, coconut sugar, and lemon juice. Stir until the rhubarb is coated.
Spoon filling into 8 ounce oven safe ramekins until 3/4 full. Fill the rest of the way with the topping.
Place ramekins onto a parchment or foil-lined baking sheet and bake for 45 minutes.