Heat oven to 190C/fan 170C/gas 5. Butter two 20cm sandwich tins and line with non-stick baking paper. In a large bowl, beat all the cake ingredients together until you have a smooth, soft batter.
Divide the mixture between the tins, smooth
the surface with a spatula or the back of a spoon, then bake for about 20 mins until golden and the cake springs back when pressed. Turn onto a cooling rack and leave to cool completely.
To make the buttercream filling, beat the butter until smooth and creamy, then gradually beat in icing sugar. Peel and cut out segments of an orange with a knife, then set aside for the decoration. Squeeze the juice from what’s left and add to the butter cream.
Bring the juice of another orange, the caster sugar and the sprigs of rosemary to the boil in a small saucepan. Strain the syrup over the tops of the warm sponges – reserving the rosemary, cool, then sandwich together with the buttercream.
Decorate the cake with the orange segments and candied rosemary. Keep in an airtight container and eat within 2 days.