Preheat oven to 350°F. Place paper liners in 9 cups of a 12-cup muffin pan.
In a bowl, whisk together cocoa and boiling water until smooth.
In a bowl, combine flour, baking powder, and baking soda.
In a bowl using an electric mixer, beat eggs and Splenda Sweetener on high speed for 5 minutes. Add butter and vanilla extract; beat on medium speed until blended.
Add cocoa mixture; mix on low speed until blended.
Add about half the flour mixture; mix on low speed to blend. Add buttermilk and mix to blend, then add remaining flour mixture and blend on low speed until smooth.
Mix in cherries and walnuts. Spoon batter into prepared muffin pan, filling cups about ¾ full.
Bake for 25–30 minutes or until a wooden pick inserted into center of a muffin comes out clean. Remove from pans immediately and cool on a wire rack.