Preheat oven to 350F. Line muffin pan with cupcake liners or place baking cups on a baking sheet. Set aside.
In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda and salt.
In a medium bowl, whisk the eggs, sugar, brown sugar, canola oil and vanilla extract until combined. Pour half into the flour mixture, then 1/4 cup buttermilk and whisk for a few seconds. Add remaining wet ingredients and buttermilk, stirring until combined. Don't over mix!
Spoon into cupcake liners or baking cups, filling halfway full. Bake for 16 – 21 minutes, until a toothpick inserted into the middle comes out clean. Let cool.
In the bowl of a standing mixer fitted with the paddle attachment, beat butter on medium until smooth and creamy. Add marshmallow creme and mix until combined. Add powdered sugar and vanilla and mix until combined. Use a rubber spatula to scrape down the sides and bottom of the bowl as necessary.
Transfer to a piping bag fitted with your favorite decorating tip. Frost the cooled cupcakes and then sprinkle with mini marshmallows.