Heat the oven to 180C fan. Grease a square baking tin and line with baking paper.
Chop the dates, put them in a bowl with the boiling water and bicarbonate of soda, and set aside for 10 minutes. They will fizz up and soften.
Beat the 75g butter and 150g sugar together until pale and fluffy, then beat in the egg.
Fold in the flour, then stir in the date mixture including all the liquid. It will look worryingly wet. That is right.
Pour into the tin and bake for 30 to 35 minutes until a skewer comes out clean.
Meanwhile, make the sauce: melt the 50g butter, 100g sugar and milk together in a small pan, then bubble for 3 to 4 minutes until it thickens slightly.
Poke holes all over the warm pudding and pour over half the sauce. Serve in squares with the rest of the sauce and custard or ice cream.