Preheat the oven to 190 °C. Grease and line 2 x 22–23-cm cake tins with baking paper.

Using an electric mixer, cream the butter and castor sugar for 3 minutes. Add the eggs, one at a time, ensuring that each is fully incorporated before the next addition. Add the orange zest and vanilla extract. Continue to whip until the mixture is light and fluffy.

In a separate bowl, sift together the flour and salt.

Add the flour and milk to the creamed butter and sugar in 3 parts. Be careful not to overmix. Divide the cake mixture evenly between the 2 tins, then bake for 30–35 minutes. The cakes should be springy to the touch, and a knife inserted into the centre should come out clean. Leave to cool in the tins.

Put all the buttercream icing ingredients in the bowl of an electric mixer and beat until pale and fluffy. Use a third of the icing to sandwich the two cake layers together, and the remainder to ice the top and sides. Decorate with zested orange peel if desired.