I use this on cakes of all kinds. It's very simple and you can use it plain or I use Jello Gelatin powder to flavor it and color it! This always turns out great! I use it on jello poke cakes a lot and it's a great addition!
Make sure that the butter and cream cheese are room temperature. If they aren't, the frosting will not be creamy.
In an electric mixer, using the whisk attachment, combine cream cheese and butter. Scrape the sides as needed. Add vanilla and milk and slowly incorporate until smooth.
Add powdered sugar by the tablespoon while the mixer is on a low speed. Once all the powdered sugar is incorporated, whip for at least 1 minutes. Scrape bowl of mixer as needed. Optional: Add the jello powder at this time, first starting on low speed and increasing speed to incorporate.
Spread on cake and refrigerate for a few hours to let the butter and cream cheese set up fully.