Preheat oven to 350° F. Prepare a 12-cup muffin tin with paper liners and set aside.
In a medium bowl, whisk together flour, baking powder and salt. Set aside.
In the bowl of a stand mixer, beat butter until smooth and light in color, about 3 minutes. With the mixer on low, add the maple syrup in a steady stream. Increase speed to medium and mix until well combined, about 3 more minutes. Add the egg and the egg yolk one at a time and mix until incorporated. Scrape down the bottom and sides of the bowl if necessary.
With the mixer on low, add half the flour mixture and mix just until combined. Add the buttermilk, then the remaining flour mixture. Do not overmix.
Transfer batter to prepared muffin tin filling each cup about three-quarters full. Bake for 23 to 25 minutes or until a toothpick inserted into a cupcake comes out with little to no crumbs. Let the cupcakes cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Fill a saucepan with 1 inch of water. Place over low heat and bring to a simmer.
Add the eggs whites, sugar, cream of tartar and salt to the metal bowl of a stand mixer. Place bowl over simmering water and whisk the mixture constantly until the egg whites are warm to the touch, about 3 to 4 minutes.
Transfer bowl to a stand mixer with whisk attachment. Whisk the mixture on medium-high speed until stiff peaks form, about 8 minutes. Add the vanilla and mix for another 15 seconds until incorporated.
Transfer frosting to a piping bag fitted with desired tip. Frost the cooled cupcakes and sprinkle the marshmallow and graham cereal on top. Store cupcakes in an airtight container the refrigerator for up to 3 days.