Beat the butter and cream cheese on medium-high for 2-3 minutes until it is fluffy.
Turn the speed down and add the pistachio pudding and continue to mix.
With the mixer at a lower speed, continue to add the buttercream 1/4 cup at a time.
Once all the sugar has been added, turn up the mixer speed and mix for another 3-4 minutes.
Transfer to a piping bag with a tip and frost cupcakes.