Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). Grease a 9×13-inch (23×33-centimeter) pan with one tablespoon of margarine and set it aside.

Toss the dates in a small bowl with one teaspoon of flour to prevent the dates from sticking to your knife while you’re chopping them. On a cutting board, use a large knife to chop the dates into 1/4 to 1/2-inch (six-millimeter to 1.25-centimeter) pieces. Put the dates back in the bowl and pour the boiling water on top. Add the baking soda and vanilla and stir. Set aside.

In the bowl of a stand mixer or in a large bowl, beat the margarine and brown sugar on medium speed until creamed together. Add the eggs and beat for one minute, stopping to scrape down the bowl once, and beat until thick and creamy. Add 1/4 cup (30 grams) flour and baking powder and mix on low speed. Add 1/2 cup (65 grams) flour and mix until just mixed in. Add the last 1/2 cup (65 grams) flour and mix until just combined. Add the date-and water mixture and mix well with a silicone spatula or wooden spoon.

Pour the mixture into the greased pan and bake for 20 to 22 minutes, or until a toothpick inserted in the center comes out clean. Check the cake for doneness, starting after about 19 minutes of baking – you want to remove the cake precisely when it is just baked and not a minute longer.

In a small saucepan, cook the margarine and brown sugar over medium heat until the margarine is melted, stirring often. Mix in the cream.

Cook the mixture until it boils and continue cooking it for three more minutes. Reduce the heat to low and cook the mixture for another two minutes.

When the pudding is baked, remove it from the oven and raise the oven temperature to broil. Use a lollipop stick or skewer to poke holes all over the cake. Pour the toffee over the top of the cake and put it in the oven for one minute, allowing the mixture to bubble.

To serve, cut the warm pudding into squares or scoop it out of the pan with a spoon. Store covered at room temperature for up to five days or freeze for up to three months. Reheat to serve.