Prep the dates: Preheat your oven to 350°F (175°C) and grease a 9x9 inch baking dish. Chop the dates finely and place them in a heatproof bowl. Sprinkle the baking soda over the dates and pour the boiling water on top. Let this mixture sit for 15 minutes to soften, then mash it slightly with a fork. Mix the wet ingredients: In a large mixing bowl, cream the softened butter and dark brown sugar together until light and fluffy. Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.Combine wet and dry ingredients: Gently fold the flour, baking powder, and salt into the butter mixture. Be careful not to overmix. Finally, stir in the soaked date mixture, including all the liquid. The batter will seem quite thin, but this is exactly what you want for a moist sticky toffee pudding cake.Bake the sponge: Pour the batter into your prepared baking dish. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.Create the toffee sauce: While the cake bakes, combine 1/2 cup butter, 1 cup heavy cream, and 1 cup brown sugar in a saucepan over medium heat. Bring to a gentle boil, stirring constantly, and let it simmer for 3 to 5 minutes until it thickens into a rich toffee sauce.Soak and serve: Once the cake comes out of the oven, use a skewer to poke holes all over the top. Pour half of the warm toffee sauce over the hot cake, allowing it to soak in. Serve warm with the remaining sauce.