Move rack to center of oven. Preheat oven to 375°F. Thoroughly grease a 12-cup muffin tin and set aside.
In a large saucepan, melt 1/4 cup chocolate chips and butter over medium heat, stirring constantly, until mixture is nearly smooth. Remove from heat. In the following order, add granulated sugar and egg; then cocoa powder, baking powder and salt at once; and then milk, whisking after each addition until blended. Whisk in flour until nearly smooth. Stir in walnuts, cherries and remaining 1/2 cup chocolate chips.
Spoon batter into prepared muffin cups. Bake until springy to touch, about 20 minutes. Let cool in pan for 15 minutes.
Scrape down sides of each muffin cup with a knife to release muffins and then unmold. Let cool slightly on racks. If desired, dust tops with a little confectioners' sugar before serving.