Preaheat oven to 350. Butter standard muffin tins. Whisk the milk, sour cream and eggs together in a mixing bowl and set aside. In the bowl of an electric mixer fitted with a paddle or using a wooden spoon in a mixing bowl, combine the dry ingredients. Cut in the butter until the mixture resembles cornmeal. Pour the sour cream mixture into the butter and flour mixture and mix until the dough comes together. Do not over mix. Gently fold the orange juice, zest and orange liqueur into the batter. Using an ice cream scoop, fill the muffin cups 3/4 full. Sprinkle the tops with a little brown sugar, about 1/2 to 3/4 teaspoon per muffin. Bake in the center of the oven for about 25 minutes. Immediately turn muffins out of the tins onto a cooling rack.