Transfer the dates into a saucepan together with the water, the butter and the salt. Now bring it to a boil and once it’s boiling turn of the heat and let it sit for 5 minutes. Then puree it with a hand blender. After that mix the flour with the sugar, the baking powder, the eggs and the date puree. Then mix it till smooth. Now pour it into a baking tin and bake it at 170 degrees Celsius for 40 minutes. Then cover it with a plate and let it rest in your fridge for at least 6 hours.