Preheat the oven to 350°F. Line a cupcake pan with paper liners.
In a large bowl, combine the cake mix, eggs, extra yolk, melted butter, and milk. Mix until smooth and fully combined, about 1–2 minutes.
Fill each liner about ⅔ full. Bake according to the package time, usually 18–22 minutes, until the centers spring back lightly when touched. Let cool completely before frosting.
For the buttercream, beat the softened butter until smooth and creamy, about 2 minutes. Gradually add powdered sugar. Mix in vanilla. Add heavy cream one tablespoon at a time until the frosting is fluffy and spreadable. Beat for another 2–3 minutes until light and airy.
Pipe or spread frosting onto cooled cupcakes. Finish with pastel sprinkles and baby toppers.