Step 1: Preheat your oven to 180°C (350°F) and grease a 20cm square baking dish. Place the chopped dates in a bowl, sprinkle with baking soda, and pour the boiling water over them. Let sit for 10-15 minutes.
Step 2: In a large mixing bowl, cream together the softened butter and dark muscovado sugar until the mixture is light and fluffy.
Step 3: Add the eggs one at a time to the butter mixture, beating well after each addition. Stir in the vanilla extract.
Step 4: Sift together the flour, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture until just combined.
Step 5: Stir in the date mixture (including the soaking liquid) until evenly distributed. Pour the batter into the prepared baking dish.
Step 6: Bake for 30-35 minutes, or until a skewer inserted into the center comes out clean. While the cake is baking, prepare the sauce by melting butter, sugar, and treacle in a saucepan over medium heat, then whisking in the heavy cream and simmering for 3-5 minutes.
Step 7: Once the cake is out of the oven, prick holes across the surface and pour half of the warm sauce over it. Serve the remaining sauce on the side.