Heat oven to 180C/160C fan/gas 4 and line a 12-hole muffin tin with cases. In a large mixing bowl beat together the butter, 175g of the sugar and the vanilla extract until light and fluffy. Beat in the whole egg, followed by the 4 yolks, before briefly beating in the flour, baking powder and milk until smooth. Divide two-thirds of the mix evenly between the cases, then top the centre of each with 1 tsp jam. Divide remaining cake mix over the top, using a wet finger to smooth it in place so no jam is visible. Bake for 18-22 mins until a skewer poked into the centre comes out clean. Wait until cool enough to handle, then transfer the cakes to a flat baking sheet.