1. Preheat the oven to 180°C, gas mark 4. Butter a 1 litre, shallow baking dish.
2. Put the flour, baking powder and a pinch of salt in a large bowl. In another bowl, whisk together the buttermilk, butter, egg yolks, sugar and lemon zest, then stir into the flour mixture.
3. Whisk the egg whites into stiff peaks. Using a metal spoon, fold half into the batter, then gently fold in the rest.
4. Pour the batter into the baking dish. Sit it in a larger baking dish, then pour boiling water into the larger dish until it comes halfway up the sides of the smaller dish. Cook in the oven till puffy and golden, about 30-35 minutes. Let cool for 10 minutes before serving.
5. Meanwhile, prepare the apricots. Preheat the grill to high. Lay the fruit cut-side up in an ovenproof dish. Mix the vanilla seeds with the sugar and sprinkle over the apricots. Drizzle with honey and dot with the butter. Grill for 5-7 minutes; serve with the pudding.