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