In a sauce pan, melt the butter until bubbling. Add shallots and garlic; cook for 3 minutes or until the aromatics are soft and sweet (watch to make sure you don't burn the shallots or garlic). Add the balsamic vinegar; bring to a boil. Once boiling, reduce the temperature. Simmer for 15 minutes until the sauce is reduced in half. The balsamic should be thick, glossy and coat the back of a spoon.