Put the dates in a bowl and pour in 300ml boiling water. Leave to soak for 30 mins. Meanwhile, make the sauce by tipping all the ingredients, a big pinch of salt and 300ml boiling water into a pan, then whisk over a medium heat and simmer for 2 mins. Pour into a jug and leave to cool slightly. Once the dates are soaked, use a hand blender to blitz the dates and water until you have smooth paste. Leave to cool slightly. Butter a deep 35 x 25cm dish.
Heat oven to 180C/160C fan/gas 4. Beat together the butter and sugar for 3 mins until smooth. Add the eggs, one at a time, beating between each addition. Fold in the flour, bicarb and baking powder along with a pinch of salt. Once combined, mix in the date purée, then fold in the dark chocolate. Pour the sponge batter into the prepared dish and spread out evenly using the back of a spoon. Pour the chocolate sauce evenly over the top of the batter.
Bake in the oven for 30-35 mins until risen, then leave to rest for 2 mins. Serve warm with a big scoop of ice cream or warm custard.