Vanilla-y, rainbowy homemade funfetti cupcakes with a creamy marshmallow buttercream frosting. This recipe is for a high-altitude cake (about 5,000 ft.), but if you live at sea-level, you can try 1 1/2 tsp. of baking powder and only 1/2 c. milk. Bake the cakes at 350° for 18-22 minutes. *I have not made this cake at sea-level for a few years, but this recipe worked fine for me (with the above) changes when I used to live in Las Vegas!
Preheat the oven to 375° F. Line two cupcake tins with 18 liners. Whisk together the flour, baking powder, and salt in a small bowl and set aside. In a liquid measuring cup, mix together the whole milk with the sour cream (or Greek yogurt) and set aside. Then, mix the sprinkles with a few pinches of flour in a small bowl until they are lightly coated.
Using a mixer, cream the butter and sugar on medium-high until light and fluffy, about 4 minutes. Add the eggs one at a time until incorporated. Then, mix in the imitation vanilla extract and oil.
Reduce the mixer to low, and add half the flour mixture followed by half the milk mixture until just combined. Repeat with remaining flour and milk mixtures. Do not overmix. Gently stir in the sprinkles until evenly combined. Divide the batter evenly between the cupcake liners.
Bake for 14-16 minutes, or until the cupcakes are just baked and when a toothpick inserted into the center of the cakes has a few crumbs on it. Let the cupcakes cool for a few minutes in the pan, and then completely cool on a wire rack.
In a medium bowl, mix together the butter and marshmallow fluff until creamy. Add the powdered sugar, one cup at a time, until thoroughly mixed. Add the imitation vanilla extract, 2 Tbs. of milk, and a pinch of salt and mix on medium-high until whipped, about 5 minutes. Add more milk or powdered sugar, 1 Tbs. at a time, until you reach your desired consistency (if necessary).
Pipe the frosting onto the cooled cupcakes. Decorate with sprinkles or other confections as desired. Store leftovers in an airtight container for up to 3 days. Enjoy!