Preheat the oven to 180°C / 160°C fan/ gas 4 and fill a 12 muffin tray with cases.
In a bowl, beat the eggs for a couple of minutes until light and fluffy.
Add in the milk and the vegetable oil beat until just combined then add the caster sugar and whisk until you have a smooth batter.
Fold in the sieved self-raising flour and salt. You don't want to over mix the mixture as this can lead to a muffin with a tough texture.
Then fold in the zest of the lemon.
Split the mixture between the 12 muffin cases, but don't fill above ¾ of the cases.
Place in the oven to bake for 15-20 minutes until golden brown and a skewer inserted into the middle of each cake comes out clean. Then place on a wired cooling rack.
To make the buttercream, in a bowl, beat together the softened butter and icing sugar.
Then add the lemon juice with the zest, to loose the mixture slightly.
Place the mixture into a piping bag.
Pipe the mixture in top of the fully cooled muffins.
Decorate with a fresh slice of lemon before serving.