Place chopped dates in a bowl and pour boiling water over them. Stir in baking soda and let sit for 10–15 minutes to soften.
Preheat oven to 350°F (175°C). Grease an 8-inch baking dish. In a mixing bowl, cream butter and brown sugar until fluffy. Add eggs one at a time, then mix in vanilla.
In a separate bowl, whisk together flour, baking powder, and salt. Add the dry ingredients to the butter mixture and stir to combine. Finally, stir in the softened date mixture (including the liquid).
Pour the batter into the prepared baking dish and bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean. While baking, prepare the toffee sauce.
In a saucepan over medium heat, melt butter and add brown sugar, stirring until dissolved. Slowly stir in heavy cream, vanilla, and a pinch of salt. Simmer for 5–7 minutes until thickened slightly.