Preheat oven to 375ºF. Grease a 12-cup muffin tin. Combine flour, brown sugar, and cinnamon. Work butter into the dry ingredients with a pastry blender or fork until butter is the size of small peas. Stir in oats. Set aside. Combine dry ingredients in a medium mixing bowl. Combine milk, oil, egg and maple extract in a separate bowl. Add liquid ingredients to dry, mixing just until dry ingredients are moist. Stir in cranberries and walnuts. Fill each muffin cup 2/3 of the way full with batter. Sprinkle topping evenly over each muffin. Bake for 25 minutes or until golden brown. Cool 5 minutes in pan; remove to a wire rack to cool completely. Makes 12 muffins.