Preheat the oven to 175℃, grease a 20cm square baking tin with butter.
Chop the medjool dates into fine pieces. Soak them in 150 ml of boiling water for 15 minutes.
Meanwhile, add the butter and soft brown sugar into a large mixing bowl. Use an electric whisk to beat it together until it's creamy.
Add the eggs and beat until combined. Next, add the black treacle and use a spoon or spatula to beat it together.
Tip in the self raising flour and baking powder, fold it together until it's well combined.
Pour in the milk and mix it well.
Now the dates have soaked up all the water, use a fork to mash the dates almost until it is a paste. Fold them through the sponge batter.
Pour the mixture into the greased up baking tray. Bake it in the oven for 50 minutes, or until a skewer poked in the middle comes out clean.