Heat the butter in a saucepan over medium-low heat. Add in the shallots and garlic, cooking until soft and slightly golden and stirring occasionally, about 8 to 10 minutes. Stir in the tomato paste and chipotle powder and cook for another 5 minutes. Add in the tomatoes, sherry, chicken stock, brown sugar, basil, salt and pepper, then bring the mixture to a boil. Reduce to a simmer and cook for 20 minutes.