Preheat oven to 350 degrees. Butter a 10-inch round or square baking dish. Sift the flour and baking powder onto a sheet of waxed paper. Chop the dates fine. Place in a small bowl and add the boiling water and baking soda; set aside. In a bowl of electric mixer beat the butter and sugar until light and fluffy. Add the egg and vanilla; beat until blended. Gradually beat in the flour mixture. Add the date mixture to the batter and fold until blended with a rubber spatula. Pour into the prepared baking dish. Bake until pudding is set and firm on top, about 35 minutes. Remove from oven to a wire rack.
Toffee Sauce: Combine the 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 about 1/3 cup of the sauce over the pudding. Spread evenly on top. Place pudding under the broiler until the topping is bubbly, about 1 minute. Serve immediately spooned into dessert bowls. Drizzle with toffee sauce and top with a spoonful of whipped cream.