Heat oven to 180C/160C fan/gas 4. You will need 2 x 20cm loose-bottomed sandwich tins, greased and bases lined with baking parchment. Measure all the cake ingredients into a large bowl (reserve a little orange zest for decoration) and beat with a wooden spoon or electric hand mixer until combined and smooth.
Divide evenly between the 2 tins. Bake for 20-25 mins or until well risen, lightly golden and shrinking away from the sides of the tins. After 5 mins, remove from the tins and leave to cool on a wire rack.
To make the icing, put the butter and icing sugar into a bowl and mix with an electric hand mixer until light and fluffy. Stir in the orange zest.
Remove the paper from the cakes. Sit 1 cake upside down on a plate. Make the glaze by putting the caster sugar and orange juice into a saucepan, stirring over a low heat until the sugar has dissolved. Boil until reduced by half, then brush half on the upside-down cake, using a pastry brush. Spread half the butter icing over the glazed cake. Sit the other cake on top, brush with the remaining glaze, then spread with the remaining butter icing. Scatter with the reserved orange zest. The cake is best eaten on the day, but will keep for up to 3 days in a cool place. It freezes well un-iced or filled.