Preheat oven to 175 degrees celsius (fan-forced). Grease and line a round 20cm cake tin with baking paper and set aside.
Place the butter and sugar into the Thermomix bowl and and mix for 15 seconds, Speed 5.
Add the eggs, salt, vanilla extract and self-raising flour and mix for 15 seconds, Speed 5. Scrape down the sides of the bowl, and mix for a further 10 seconds, Speed 5.
Pour the mixture into the prepared tin and bake for 25-30 minutes or until cooked when tested with a skewer.
Allow to cool in the tin for 15 minutes then move to a wire rack to cool completely.
For the buttercream
To make the buttercream, place the icing sugar into the Thermomix bowl and mix for 10 seconds, Speed 9 to sift.
Add the butter, milk, vanilla extract and food colouring to the bowl. Mix for 30 seconds, Speed 4.
Spread the buttercream frosting over the cooled cake and serve.
Notes
Keep any leftover cake in an airtight container at room temperature for up to 4 days.