Preheat oven to 350°. In a large bowl, combine first six ingredients. Whisk buttermilk, coffee, oil, eggs and vanilla in a stand mixer with the whisk attachment until blended; add the dry ingredients until combined. (Batter will be very thin.) Fill paper-lined muffin cups two-thirds full. You can use a trigger large ice cream scoop to help you measure batter evenly between the cups.
Bake 18-20 minutes or until a toothpick inserted in the center comes out clean. Cool 10 minutes before removing from pans to wire racks to cool completely.
In a double boiler over medium-high heat whisk together the egg whites, sugar, and cream of tartar. Heat about 5-7 minutes while whisking constantly until sugar is dissolved. If you rub some of the mixture between your fingers and you can no longer feel the sugar granules, it's ready.
Move the mixture immediately to a stand mixer with the whisk attachment or use a high powered hand mixer to whip the whites. Whip on highest speed for 5-7 minutes until stiff peaks are formed. Add in vanilla during the last 30 seconds.
When the cupcakes are cooled, cut a small hole in the middle to insert filling. Then pipe the marshmallow filling inside the hole with a piping bag.
Prepare the buttercream by whipping the butter and peanut butter together in a stand mixer with the whisk attachment. Mix about 3-5 minutes until the mixture lightens up several shades and looks very smooth and airy.
Add the rest of the buttercream ingredients and mix until combined. Use a piping bag and desired tip to pipe icing onto the cupcakes. Top with marshmallows for decoration (optional)
