Preheat oven to 350°F (175°C). Grease an 8×8-inch baking dish.
Place chopped dates in a heatproof bowl. Sprinkle with baking soda and pour boiling water over them. Let soak for 15 minutes, then mash slightly.
In a large bowl, cream softened butter and brown sugar. Beat in eggs and vanilla.
Add flour and baking powder, mixing gently until combined.
Fold in the softened dates and all their soaking liquid.
Pour batter into the prepared dish and bake for 30–35 minutes, until a toothpick inserted into the center comes out without wet batter.
Meanwhile, combine brown sugar, butter, and heavy cream in a saucepan over medium heat. Stir and simmer gently for approximately 3 minutes until smooth and glossy. Remove from heat and stir in vanilla.
Poke holes across the warm baked cake using a skewer.
Pour half the warm toffee sauce over the cake and allow it to soak in.
Serve warm with the remaining toffee sauce.