Preheat oven to 350 degrees. 

First, make your sweet potato puree. Take 2 small sweet potatoes and put them on a baking sheet. Pierce with a fork on all sides. Roast for approx. 45 minutes, until they are shriveling and cooked throughout. Allow them to cool for at least 30 minutes before peeling. Then use a blender or food processor to puree them. Set aside 3/4 cup of this puree for the cupcakes.

Line 12 Cupcake tins and have oven at 350 degrees. 

1. In a stand mixer, cream butter and sugars for 2 minutes.

2. Add egg and egg yolk and vanilla and cream for 1 minute more. 

3. In a separate bowl, whisk together the dry ingredients. 

4. Turn mixer to "stir". Add in half of the flour mixture. 

5. Add sweet potato puree and while it is mixing, drizzle in the buttermilk.

6. Add the remaining flour mixture.

7. Use a cupcake or ice cream scoop to divide batter evenly among 12 cupcake tins. If you make smaller cupcakes, you can make up to 14 cupcakes.

8. Bake approximately 25 minutes. Check with a toothpick for moist crumbs before removing from oven.

9. Allow to cool completely before frosting.

For Marshmallow buttercream

1. In a stand mixer, with the whisk attachment, whisk butter and marshmallow fluff until fluffy.

2. Gradually add powdered sugar (turn mixer to low until it starts to come together so it doesn't fly all over!)

3. Then add you vanilla extract and put in buttermilk a tablespoon at a time until desired consistency.

4. Add salt during final whisking, if desired, to offset the sweetness. 

5. Frost cupcakes.