Preheat oven to 350 degrees F. Make topping: Combine all of the topping ingredients in a saucepan and bring to a boil.
Remove from the heat and let cool.
Make cake: Stir the melted butter into the milk. In a mixing bowl, whisk together the flour, sugar substitute, baking powder, and cinnamon. Stir the wet ingredients into the dry ingredients with a wooden spoon; don't worry if the batter is a bit lumpy.
Divide the batter among 6 (1/2-cup) ramekins and spoon some of the topping over each cake. Bake for 45 minutes. Serve warm.