Preheat the oven to 190 ̊C/ 375 ̊F/ Gas Mark 5. Lightly grease a 12 cup muffin tin.
Put the dates in a saucepan with 150ml water.
Bring to the boil, cover and simmer gently for about 5 minutes until soft and pulpy. Remove from the heat and beat well until smooth. Leave to cool.
Put the butter and sugar in a bowl and whisk together until light and creamy. Stir in the cold date mixture and whisk in the egg and vanilla paste.
Sift the flour and baking powder on top and mix together. Gradually add all the milk to make a smooth, thick batter.
Divide the mixture equally between the cups of the muffin tins – the tins should be just over ¾ full in order to form the dome shape - smooth the tops.
Bake in the oven for 25-30 minutes until risen to a dome and just firm to the touch.
Cool for 10 minutes in the tin.
Working on one at a time, carefully inject each one generously through the side with the salted caramel cupcake centre, making sure the nozzle is inserted fully into the centre of the duffins. Prop the duffins on their sides back in the muffin tins and leave for 10 minutes to allow the filling to settle, then transfer to a wire rack.
To decorate, while the muffins are still warm, put the sugar on a plate. Working on one muffin at a time, brush all over with melted butter and roll in the sugar to coat. Put back on the wire rack.
Your duffins are now ready to serve warm or allow to cool completely.