Preheat the oven to 180C, fan 160C, gas 4 and grease well 8 holes from a 12-hole muffin tin.
Start by adding all the dry ingredients in a bowl; flour, sugar, mixed spice and salt. Then slowly stir in the oil, eggs, buttermilk and vanilla, and fold carefully until just combined.
Using either a piping bag or spoon, divide half the mixture into each muffin hole, then add a teaspoon of Jubilee jam into the centre. Cover the jam with the remainder of the batter mix and place in the oven for 25 minutes, or until golden and firm to the touch.
Remove from the oven and place on to a wire rack. When the muffins are cool enough to handle, brush the surface of each with the melted butter, then roll them in the remaining sugar to evenly coat.