Step #1 In a bowl, whisk the flour & next 5 ingredients togehter until well mixed; set aside.
Step #2 In a big bowl, whisk the egg until lightly beaten; then whisk in the yogurt until smooth.
Step #3 Whisk in the milk, melted butter, & vanilla.
Step #4 Using a wooden spoon, stir in the the chocolate chips & marshmallows; then stir in the flour mixture until the chips & marshmallows are evenly distributed in the batter (don't over mix).
Step #5 Fill muffin c that have been sprayed with nonstick cooking spray 3/4 full.
Step #6 Bake in a 400F degree oven for 15-17 mins or until muffins have firm, rounded, lightly browned tops; a pick should come out with a few crumbs attached.
Step #7 Place pan on a wire rack to cool 10 mins.
Step #8 Turn out muffins; cool on wire rack 5 mins.
Step #9 Cool muffins completely before storing in an air-tight container or freezing them in freezer bags.