These muffins are absolutely amazing! The recipe uses a combination of butter and oil to provided both flavor and moisture, while buttermilk adds richness and a slight tang. The muffins crumb is moist and tender, yet sturdy enough to support the blueberries. Best of all, they are packed full of flavor...every bite bursting with blueberry yumminess either from fresh whole blueberries or a blueberry sauce. To make the berry sauce, half of the fresh blueberries are cooked down into thick jam-like sauce to concentrate their flavor and reduce excess moisture. The berry sauce is then swirled into each batter-filled cup to spread concentrated berry flavor throughout each muffin just before baking. The berry sauce is an additional step not found in other recipes, but it's also a step that elevates this recipe above those other recipes.