Preheat the oven to 180°C conventional, 160°C fan (356°F conventional, 320° fan).
Grease a 23x23 cm (9x9 inch) baking dish with butter, set aside.
Place the roughly chopped dates in a heatproof bowl. Pour on 1 cup boiling water and bicarbonates of soda. Set aside for 10 minutes while we make the cake batter.
Using a stand or hand mixer on medium-high, start beating the softened butter with the dark brown sugar until fluffy and well incorporated (about 4-5 min). If you still have lumps of sugar use a spatula to flatten them out.
Add in your eggs one at a time, beating after each addition. Scrape the bowl to make sure the mixture is well incorporated.
Lower the speed of your mixer and gradually add in the flour and the baking powder. Mix just until incorporated (do not over mix). The batter should be thick.
Use the back of a fork to mash dates into the water, a loose paste should form. It is fine to still have a couple of bigger pieces.
Fold the date mixture into the cake batter using a spatula until well incorporated.
Pour batter into the prepared baking dish making sure you use a spatula to scrape all the batter out of the bowl. Bake for 30-35 minutes or until a toothpick inserted into the center of the cake comes out clean. Do not overbake we want the cake to be very moist.