Preheat oven to 325 degrees Fahrenheit.
Cream together the butter and caster sugar in a stand mixer for 10 minutes. Scrape the mixture down the side of the bowl at least once during mixing.
Add one egg then mix for 3 minutes. Then add the next egg and repeat until very light and fluffy.
Combine the flour, cinnamon, and chipotle powder in a separate bowl.
In a separate small bowl, add the chopped dates and boiling water, then mix the baking soda together with the mixture. (You can blend this mixture with a stick blender now if you like a darker sponge with no bits.)
Add all of the flour mixture and the date mixture to the egg, butter, and sugar mix then fold carefully with a spoon. Be careful not to over mix.
Pour the mixture into a square cake tin and cook in the center of the oven for 40 minutes.