In a medium bowl, combine cake mix and root beer. Spoon mixture into cupcake liners.
Add water to the Instant Pot® and insert steam rack. Place six cupcake liners on steam rack. Lock lid.
Press the Manual or Pressure Cook button and adjust time to 9 minutes. When timer beeps, quick-release pressure until float valve drops. Unlock lid. Transfer cupcakes to a cooling rack. Repeat cooking process with remaining six cupcake liners.
To make buttercream, cream together vanilla buttercream ingredients in a medium mixing bowl. If buttercream is too loose, add a little more confectioners’ sugar. If buttercream is too thick, add a little more milk.
Let cupcakes cool for at least 30 minutes until they reach room temperature, then spread buttercream on cooled cupcakes. Serve.