My 12yo daughter, who loves to experiment with baking in the kitchen, tried to improvise a cupcake recipe today.  She was disappointed in the result (see photo). Ingredients she used were, (not sure of amounts, I wasn't looking over her shoulder) flour, sugar, salt, baking powder, eggs, oil, butter, and some melted marshmallows in melted butter. I'm not very good in the kitchen and I don't know what to tell her went wrong.  They puffed up and then deflated. Taste good  quite chewy. Suggestions for her? Thanks!

It sounds likely that she used too high a proportion of fats with the butter and oil both.  I have no idea how marshmallows melted in butter would effect the baking.  I've only ever used melted marshmallows in fondant and Rice Krispy Treats.  

Maybe she should use good, proven recipes until she is more familiar with the science of baking.  I, after a lifetime of baking, am reluctant to deviate too far outside the normal proportion for cakes and cupcakes.  Here is a good recipe and video with which to start.

https://natashaskitchen.com/perfect-vanilla-cupcake-recipe/