Blend all ingredients in a Thermomix 10 sec/speed 5, or blend in a mixer or food processor on half speed, until creamy.
Scrape down sides, insert butterfly whisk (or whisk attachment), and mix 1 min/speed 4 until mixture is very creamy and has a ‘whipped’ texture. Stop and scrape down side of bowl as needed.
Spread on top of cake, making scallops with the knife. Decorate as above.
Notes
Double this recipe if making a layer cake.
Store cake in the fridge for up to 1 week, or freeze well-wrapped (iced, but without decorations) for up to 2 months.