Preheat your oven to 350°F.
Prepare your baking pan by greasing it with butter or lining it with parchment paper.
Combine the chopped dates and boiling water in a bowl.
Add the baking soda and set aside for the dates to soften.
In a separate bowl, cream together the unsalted butter and dark brown sugar until light and fluffy.
Beat in the egg until well combined.
Fold in the all-purpose flour and baking powder gently.
Add the sliced banana and pureed dates.
Mix until everything is just incorporated.
Pour the batter into the prepared baking pan.
Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
While the cake bakes, make the toffee sauce by heating the granulated sugar and heavy cream in a saucepan over medium heat.
Stir until the sugar dissolves and the sauce becomes smooth.