Preheat oven to 350°F. Butter a 9-inch square baking dish.
Sift flour and baking powder into a small bowl. Set aside.
Chop the dates finely. Place in a small bowl and add the boiling water and baking soda; stir and set aside. In the bowl of an electric mixer, beat the butter and sugar until light. 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 30 minutes. Remove from oven to a wire rack.
For the 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 2/3 to 1 cup of the sauce over the pudding. Spread evenly on top. Place pudding under the broiler until the topping is bubbly, only about 1 minute. Serve immediately. Drizzle with toffee sauce and top with a spoonful of rum-flavored whipped cream.
High altitude: reduce baking powder and baking soda by 1/8 teaspoon, increase boiling water by 2 to 3 tablespoons, and decrease granulated sugar by around 1/2 to 3/4 teaspoon.
Make ahead: Pudding can be baked earlier in the day, covered and stored at room temperature. Make toffee sauce just before serving.