If you are using stone fruit, cut the flesh in half along the seams, then twist and take out the stones. If your fruit is unripe, you may need to use a small knife to prise or cut the stone out; if that shreds one of the fruit halves into smaller pieces, it’s not a big deal, you can still use them.Preheat the oven to 180°C fan. Line a 20cm cake tin with a circle of baking parchment, then either lightly oil it or butter it heavily. The butter will add richness and flavour, so it really is up to you which one you pick.Pound the cardamom seeds with the 1 tbsp of demerara until well ground. Sprinkle over the base of the prepared tin.Mix the flour with the baking powder and sift them, if you have patience. Mix in 1 tsp salt.Whisk the eggs in a mixer, then gradually add the 200g of demerara sugar and whisk until very pale and fluffy. Gradually add the kefir or ryazhanka, whisking on a slightly lower speed, then trickle in the 100ml of oil and add the vanilla. The mix might look grainy and split, but it will be all OK once you add the flour.Fold in the flour and baking powder with a spatula, making sure there are no dry pockets of flour. Place the fruit into the batter, then scrape it into the sugared-and-cardamomed tin, smoothing it over on top.Put it into the oven and cook for 45 minutes. Check by inserting a skewer, which should come out with no cake mixture stuck to it. But remember the fruit is wet, so it’s OK for the skewer to come out moist. The cake will continue cooking for a bit when you take it out, so be careful not to dry it out.