Preheat the oven to 180'C.
Butter your cake tin and line with baking parchment.
Split the vanilla pod in half and scrape out the seeds. Zest the lemon.
Whisk together the butter, sugar, lemon zest and vanilla seeds until light and creamy. One by one, whisk in the eggs until well combined. Sift in the flour and the baking powder, folding the mixture together. Gently fold in the Madeira and then pour the mixture into the baking tin.
Bake for 30 minutes and then check the centre of the cake with a skewer. If the skewers come out clean, the cake is ready. If not, cook for a further 8-10 minutes.
Allow to cool before serving.
«Take care to fold in the flour rather than beating it in. This will keep the cake light and stop it being dense.»