Melt butter in a large pan together with the sugar and treacle.
Allow to cool a little whilst getting on with weighing the flour and chopping the chocolate.
Mix in sour milk and stir until smooth.
Beat in the egg.
Stir in the marrow and ginger jam.
Sift in the dry ingredients and stir to combine.
Stir in the chocolate.
Spoon mixture into 12 cupcake cases, drop 2 pieces of chocolate on each one and bake at 180℃ (350℉, Gas 4) for 20 minutes.
Turn out onto a rack and leave to cool.
Buttercream
Cream butter with icing sugar until almost white.
Beat in 2 tsp lemon juice and the jam syrup.
Spoon into a piping bag and using a star nozzle pipe the buttercream onto the cupcakes.
Decorate with ginger and chopped chocolate if desired.
Notes
To make a cheat's version of sour milk, stir a tsp of lemon juice into the milk and leave for a few minutes to thicken.Please note: calories and other nutritional information are per serving. They're approximate and will depend on exact ingredients used.