Preheat an oven to 350 F. Line a 12-count cupcake pan with liners. Combine the all-purpose flour through the baking soda in a mixing bowl.
Beat the eggs and both vanillas in a stand mixer fitted with a paddle attachment. Add in the avocado oil and beat well. Then add in the whole milk and champagne. Beat to combine.
Add in the dry ingredients and beat until just combined.
Fill the liners ⅔ of the way full. Bake for 14-15 minutes until lightly golden on the very edges and cooked through. The cupcakes should still look pale in color.
Combine the ingredients for the buttercream. Beat for 7-10 minutes until extremely light and fluffy. Fill a large piping bag fitted with a star tip (Wilton # 826) with the buttercream. Pipe the frosting onto the cupcakes in a swirl pattern ending in the middle. Pull the piping tip straight up to make a point.
Tips
Be sure to use room temperature butter and beat the ingredients for the entire length of time for the fluffiest buttercream frosting! If not using salted butter, be sure to add a pinch of salt to make the frosting extra delicious.