Cream butter (or, margarine) and sugar until well blended.
Add eggs, vanilla extract, sour cream and milk and mix on high until very light and fluffy.
Add dry ingredients to the fluffy mixture, and mix on low (or, by hand) until all ingredients incorporate well. Do not over-mix.
Gently fold in cranberries.
Line muffin tray with muffin cups or spray with cooking spray. Fill each cup 2/3 full with batter. Sprinkle with sugar (optional).
Bake at 400 degrees for 15 minutes or until the tops start to turn light golden brown.
Remove from the oven and repeat steps 5 and 6 until batter is exhausted. Recipe makes 18 muffins (but can vary based on the size of your muffins and the muffin tin you use).
Enjoy your muffins!