Place condensed milk and 1 tbsp butter in a sauce pan over medium heat.
Stir until the mixture thickens (10-15 minutes). The mixture should be thick so you can see the bottom of the pan when running a wooden spoon though it.
Let it cool completely.
Grease your hands with some butter and make 1 inch - 2,5 cm sized balls.
Roll them in sugar or sprinkles and serve them in mini cupcake liners.