Preheat the oven to 170C/150C Fan/Gas 3. Butter and line a large loaf tin.
Cream together the butter, caster sugar and clementine zest – you can do this either in a stand mixer or with hand-held electric beaters.
Mix the flour and baking powder together with a pinch of salt. Add the eggs one at a time to the sugar and butter mixture, sprinkling over a couple of tablespoons of flour with each addition and mixing well.
Fold in the rest of the flour followed by the clementine juice – the texture will be slightly firmer than the dropping consistency you need for a lighter sponge.
Scrape the batter into the prepared tin and sprinkle with the caster sugar. Bake in the oven for between 55–60 minutes, or until a cake skewer comes out clean. The cake will be a rich golden-brown and may have cracked lengthways down the middle – this is normal for a madeira cake.
Remove from the oven and leave to cool in the tin before transferring to a wire rack.