Preheat oven to 375°F. Spray 8 cups of a 12-cup muffin pan with cooking spray.
In a bowl, mix together Splenda Sweetener, flour, baking powder, baking soda, and cinnamon.
In another bowl, combine buttermilk, melted butter, eggs, and orange zest. Add to flour mixture and mix gently to combine. Add cranberries and walnuts and stir just until blended.
Spoon batter into prepared muffin cups. Bake for 20–25 minutes or until a wooden pick inserted in the center comes out clean.