Preheat the oven to 350°F / 180°C. Grease and line an 8in / 20cm square pan.
Cover 280g of chopped pitted dates and 1 teaspoon of baking soda with 1 cup of boiling water. Leave for 10 minutes, then mash until porridge-like.
Beat 80g of softened unsalted butter with ¼ cup of loosely packed brown sugar until smooth. Beat in 2 eggs. Mix in 1¼ cups of all-purpose flour and 1½ teaspoons of baking powder, then the dates and their liquid.
Pour into the pan and bake for about 35 minutes, until a skewer comes out clean.
Meanwhile, heat 1 cup of tightly packed brown sugar, 1½ cups of heavy cream, ½ teaspoon of vanilla extract, and 70g of unsalted butter in a saucepan. Once melted, stir and simmer for 2 minutes.
Poke holes all over the hot cake. Pour over about one-quarter of the warm sauce and leave for 10 minutes. Serve warm with the remaining sauce.
