Spray an 8-inch (20 cm) square metal baking pan with nonstick baking spray with flour.
In a large bowl, combine the chopped dates, the baking soda and boiling water. Let it stand for about 30 minutes or until it is cooled to room temperature.
Meanwhile, preheat the oven to 350°F (180°C).
Add the brown sugar, eggs, butter and vanilla to the date mixture. Using an electric mixer on medium-high speed, beat it for 2 minutes, until blended (the mixture will appear slightly curdled). Add the flour, baking powder and salt. Beat on low speed for 1 minute. Scrape down the sides and bottom of the bowl with a spatula. Mix again for 15 to 30 seconds until blended.
Spread the batter evenly in the prepared pan and bake for 40 to 45 minutes or until a toothpick inserted in the center comes out with just a few moist crumbs attached.
Set the pan on a wire rack. Poke holes all over the top of the cake with a toothpick. Pour 1/2 cup (125 mL) of the toffee or caramel sauce over the cake. Let it cool in the pan on rack for 10 minutes.
To serve, cut the cake into squares and serve warm with the remaining sauce.
Store the cooled cake in the refrigerator in a cake keeper, or loosely wrapped in foil or plastic wrap, for up to 5 days. Warm cake, wrapped loosely in foil, in a 350°F (180°C) oven before serving.