Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners.
In a large mixing bowl, combine the cake mix, sugar, and flour. Mix until well combined.
Add water, eggs, vanilla extract, sour cream, and vanilla bean seeds (if using) to the dry mixture.
Blend the mixture on medium speed until smooth and creamy—about 2 minutes.
Evenly distribute the batter into the cupcake liners, filling each about 2/3 full.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Let the cupcakes cool in the tin for about 5 minutes before transferring to a cooling rack to cool completely.
In a mixing bowl, combine softened butter, powdered sugar, vanilla extract, whipping cream, and salt. Beat until light and fluffy.
Frost the cooled cupcakes with the buttercream frosting using a spatula or piping bag.