Preheat oven to 180°C.
Add chopped dates and boiling water to a bowl. Stir in baking soda and leave for 10 minutes to soften.
Mash the dates slightly, keeping some texture.
Cream together butter and brown sugar until combined.
Mix in vanilla and beaten egg.
Add the date mixture and stir.
Fold in flour, baking powder, and salt until just combined.
Spoon into a greased single-serve ramekin.
Bake for 18–22 minutes until risen and set but still soft and sticky.