Step #1 Position rack in center of oven; preheat your trusty oven to 400°.
Step #2 Prepare muffin tin: spray indentations/rims with cooking spray.
Step #3 Place the chocolate & butter in the top of a double boiler set over simmering water.
Step #4 Stir constantly until half the chocolate & butter is melted.
Step #5 Remove the top of the double boiler; then continuing stirring, away from the heat, until the butter & chocolate are completely smooth; cool for 10 mins.
Step #6 Meanwhile, baking powder, sugar, whisk the flour, & salt together; set aside.
Step #7 In a big bowl, slowly pour in the cooled chocolate mixture, milk, whisk the egg, & vanilla until smooth; whisking all the while, continue whisking until well mixed.
Step #8 Use a wooden spoon to stir in the half the flour mixture; carefully stir in the the cola; when foaming subsides, stir in the the remaining flour mixture & the mini marshmallows just until the flour is moistened.
Step #9 Fill the prepared tins ¾ full; bake 20 mins or until the muffins have rounded cracked tops & a pick inserted in the center comes out with a few moist crumbs attached.
Step #10 Set the pan on a wire rack to cool for 10 mins; carefully rock each muffin back & forth to release it.
Step #11 Remove muffins from the pan & cool 5 mins on the rack before serving.
Step #12 *Black Cherry Cola Muffins: substitute black cherry soda for the cola; add ½ c pitted fresh black cherries with the marshmallows; proceed as directed.
Step #13 *Dr Pepper Muffins: substitute Dr Pepper for the cola.
Step #14 *Pecan Cola Muffins: add ½ c sliced toasted pecan pieces with the flour, proceed as directed.