Preheat the oven to 180°C, fan 160°C, gas 4. Lightly brush a 12-hole muffin tin with flavourless oil. Put all the duffin ingredients – except the jam or chocolate spread – into a large mixing bowl and beat well with an electric whisk until smooth.
Add about 1 dessert spoonful of the mixture into each muffin hole, leaving about a third of the mixture for the top. Make a dip in the centre of each duffin with a teaspoon, but take care not to go too deep or the jam will come out of the bottom. Add a scant teaspoonful of jam or chocolate spread to each one. Top with a little more cake mixture and spread it out to make sure that the filling is covered.
Bake for 20-25 minutes until risen, golden and just firm to the touch.
Mix the sugar with the cinnamon, if using, and spread out on a plate. Let the cooked duffins rest in the tin for a few minutes before removing them. While still warm, roll them gently in the sugar mixture to coat. Leave to cool on a wire rack – don’t eat them straight from the oven as the jam will be very hot!
