Preheat oven to 400°F/200°C. Spray a 12 cup muffin tin with cooking spray or line with paper liners.
Melt butter and set aside. I usually do this on the stove, but you can place the butter in a 2 cup glass measuring cup and do it in the microwave. Cover with a paper towel and microwave in 20-30 second intervals until melted.
In a large bowl, combine flour, sugar, salt and baking powder. Stir with a whisk to combine.
Combine butter, oil, egg, sour cream, milk and vanilla in a 2 cup glass measuring cup or small bowl. Mix with a fork to combine, making sure that the egg is incorporated. (It is okay if the mixture looks a bit curdled.)
Pour the liquid mixture into the flour mixture and carefully stir to combine, mixing only until the dry ingredients are fully incorporated. Fold in the blueberries.
Divide batter between muffin cups. If using, combine Demerara sugar and cinnamon in a small bowl. Sprinkle on top of the muffins.
Bake for 18-22 minutes or until muffins are golden and a toothpick inserted in the center comes out clean. Cool in pan for 5 minutes, and then remove muffins from pan and cool completely on a wire rack.