Pre-heat the oven to 160C Fan/180C/350F/Gas Mark 4 and grease and line a 2lb loaf tin
Mix together the butter or baking spread and caster sugar until light and fluffy, ideally using an electric mixer
Add the eggs and vanilla extract, and whisk in until smooth
Whisk in the flour gently until fully combined
Pour the mixture into the loaf tin and bake for an hour, or until a skewer inserted in the centre comes out clean. Leave to cool fully
To make the buttercream, mix together the butter and icing sugar until they start to combine, then add the vanilla extract and milk, and mix until smooth. You can add a little more milk if the buttercream is too stiff. Use an electric mixer for the best results
Put the buttercream into a piping bag and pipe it onto the loaf cake, or spread on with a spoon or spatula
Decorate with sprinkles, if using
Store in an airtight container in a cool place and eat within 3 days. Or you can freeze it for up to 3 months