In a medium heatproof bowl, add butter, hot milk, and vanilla. Stir until the butter melts.
In a separate bowl, sift together cocoa, icing sugar, and salt.
Add the dry ingredients to the butter mixture and use a handheld electric mixer to beat the until smooth and glossy.
Use to decorate cupcakes, layer cakes and brownies or sandwich between macarons and cookies.
Notes
Why Dutch cocoa: Dutch-process cocoa is treated to reduce acidity, giving it a darker colour and smoother flavour. It blends easily into frosting and creates a rich, chocolatey taste.
Texture tip: If the frosting is too thick, add a splash more hot milk, one teaspoon at a time, until it reaches spreading consistency.
Storage: Store in an airtight container at room temperature for up to 2 days, or refrigerate for up to 1 week. Bring to room temperature and re-whip before using.
Freezing: Can be frozen for up to 3 months. Thaw overnight in the fridge, then re-whip with a touch of milk.