Heat the oven to 175 C / 350 F. Grease a 23 cm / 9-inch square baking dish and line the base with parchment. Put the chopped dates in a 1 L heatproof jug, stir in the baking soda, and pour over the boiling water. Let it stand 15 minutes, until the water is dark and the dates crush easily against the side with a spoon.

Whisk the flour, baking powder and 1/2 tsp salt together. In a 3-quart mixing bowl, beat the 85 g butter with 150 g brown sugar for 2 minutes, until paler and fluffy. Beat in the eggs one at a time, then the molasses and 1 tsp vanilla.

Fold in half the flour mixture, then half the date mixture with all its liquid. Repeat with the rest. The batter will be loose, like thick pancake batter. Stop when no flour streaks remain, then scrape it into the dish.

Bake for 30 to 35 minutes, until the top springs back and a toothpick in the center comes out with a few moist crumbs. Check at 30 minutes, since past 38 minutes the edges dry out.

In the last 10 minutes of baking, melt the 115 g butter with 200 g brown sugar in a 2-quart saucepan over medium heat, stirring about 3 minutes until the sugar dissolves. Pour in the cream, add 1/4 tsp salt, and simmer gently for 3 to 4 minutes, until the sauce coats the back of a spoon. Stir in 1 tsp vanilla off the heat.

Poke holes about 2 cm apart across the hot pudding with a toothpick. Pour about half the sauce (roughly 240 ml) over the top and let it rest in the dish for 10 minutes, so the sponge absorbs it and the squares hold their shape.

Cut into 8 squares and serve warm, with the remaining sauce rewarmed and spooned over each portion.