Heat the oven to 190°C (375°F, gas mark 5). Line a loaf tin measuring about 22 × 11 × 6 cm (8½ × 4¼ × 2½ in) with baking paper.
Sift the cornmeal, flour and baking powder into a bowl, then stir in the brown sugar and lemon zest.
Make a well in the centre and pour in the buttermilk or yoghurt, eggs and oil. Stir with a wooden spoon until the ingredients are just blended and a soft consistency forms. Do not overmix or the loaf will be heavy.
Spoon the mixture into the prepared tin and level the surface. Bake for 45 minutes, or until the loaf is well risen, firm to the touch and a skewer inserted into the centre comes out clean.
Meanwhile, put the lemon juice into a small bowl and stir in the caster sugar until it dissolves.
Remove the loaf from the tin, put it on a wire rack over a plate and leave to cool for 5 minutes.
Peel off the baking paper and prick the loaf all over with a fine skewer, inserting it deeply but not quite to the base. Spoon the sweetened lemon juice over the loaf slowly to allow it to soak in.
Put 50 ml (2 fl oz) of water into a small saucepan and bring to the boil. Put the icing sugar into a bowl and stir in 1½-2 teaspoons of the boiling water, adding just enough to make an icing that drizzles off the spoon. Using a teaspoon, drizzle the icing from side to side over the warm loaf.
Allow the loaf to cool completely on the wire rack, then slice and serve. It will keep fresh for several days stored in an airtight container.
Tip: fine cornmeal, not to be confused with instant polenta, is sold in health food stores.