Preheat oven to 350° F. Butter a 10-inch round or square baking dish.
Chop dates finely and put in a small bowl. Add boiling water and baking soda to dates and set aside.
Sift baking powder and flour and set aside.
Beat the butter and sugar with electric mixer until light and fluffy. Add egg and vanilla and beat until blended.
Gradually beat into flour mixture. Fold date mix into the batter with a rubber spatula.
Pour into prepared baking dish. Bake until pudding is set and firm on top, about 35 minutes, then set aside to cool on a wire rack.
While the pudding bakes, prepare the toffee sauce. Combine butter, heavy cream, and brown sugar in a small, heavy saucepan.
Heat to boiling, stirring constantly. Boil gently over medium low heat until mixture is thickened, about 8 minutes.
Preheat broiler. Spoon 1/3 of the sauce over pudding and spread evenly.
Place pudding under broiler until the top bubbles, about 1 minute.
Spoon into dessert bowls, drizzle with toffee sauce, and top with a dollop of whipped cream.