Preheat the oven to 200C/180C Fan/ Gas 6.
Line a muffin tray with 12 paper cases.
Sift together the flour, baking powder & salt then set aside.
Take a large bowl and put in the eggs, yogurt, milk or buttermilk, sugar, melted butter and vanilla extract. Beat together (I use electric beaters) until well combined.
Add half the flour mixture to the egg mixture and start to fold it in.
Stir the sultanas into the rest of the flour mixture then fold this into the egg mixture too. Do not over-mix; fold so that the ingredients are only just combined.
Divide the mixture between the muffin cases.
If using the demerara sugar (it will give the muffins a slightly crunchy top) sprinkle a little over each one before putting in the oven.
Bake until the muffins are risen, golden and a skewer comes out clean (approx 17 minutes).
If the tops are browning too quickly before the centre of the muffins are cooked, turn the oven down a little and continue cooking.
Transfer to a wire rack to cool.
Meanwhile, put a bowl and clean electric beaters in the fridge to go cold (this will make whipping the cream easier).
Put the double cream, sugar and vanilla extract in the cold bowl and beat until your preferred consistency is reached. Set aside.
Take the cold muffins out of their paper cases.
Cut each one in half horizontally through the middle.
Top one half with Chantilly cream and the other with 1 heaped tsp of lemon preserve.
Put each half back together.
Serve or put in the fridge until needed.
If storing in the fridge, bring back to room temperature before eating.
Use within 3 days.
Unfilled muffins can be frozen.