Preheat the oven to 180° / Gas 4. Line two bun trays with cake cases.
In a bowl, whisk together the egg and caster sugar until fluffy.
In a separate bowl sift the dry ingredients and make a well in the centre.
Melt the butter and treacle together in a small pan, cool a little and then stir into the dry ingredients, along with the egg mixture.
Bring the milk to the boil and stir into the mixture, mixing well. Add and blend in the soured cream.
Spoon the muffin mixture into the cases and bake for 20mins, until risen and spring back when pressed in the middle.
Cool on a wire rack and store in an airtight container.