Preheat the oven to 350 F (180 C) and line a muffin tin with 6 paper cups.
In a bowl, mix the egg whites, sugar, and butter with an electric mixer. Beat until the mixture is light and fluffy.
Add the buttermilk, flour, and vanilla, and mix well. Beat with an electric mixer until the mixture is light and fluffy.
Add the marshmallows.

Divide the mixture among the cups and bake for 20 to 25 minutes, or until the cupcakes are firm to the touch.
Remove them from the oven, let them cool for 5 minutes, then transfer them to a rack to cool completely.

Glaze:
In a bowl, combine the butter, mini marshmallows, and vanilla.
Microwave for 1 minute, then mix and return to the microwave until the mixture is melted.
Add the powdered sugar and mix until smooth.
Set aside 1/4 of the glaze and add a few drops of red food coloring. Mix well and let cool.
With a spatula, spread the warm glaze over the cupcakes.
With the cooled red glaze, shape small rosebuds and place on top of the glaze.