Finely chop the dates and place in a heatproof bowl. Pour over the boiling water and leave to soak for 20 minutes, until very soft. Allow to cool until lukewarm. Do not drain.
Leave the dates finely chopped for a little texture, or blend with their soaking liquid for a smoother sponge.
Preheat the oven to 360°F conventional. Lightly grease a 10×8-inch baking dish with butter.
In a large mixing bowl, beat the softened butter and brown sugar for approximately 2 minutes, until creamy and pale in color.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
In a separate bowl, whisk together the eggs and vanilla. Gradually add to the butter and sugar mixture, a little at a time, along with a spoonful of the flour mixture with each addition, mixing gently until incorporated. Mix in the black treacle.
Gently fold in the remaining flour mixture, followed by the milk and the cooled dates with all their soaking liquid. Mix until just combined, being careful not to overmix.
Transfer the batter to the prepared baking dish and spread evenly. Bake for 35–40 minutes, until the sponge is set, lightly springy, and a skewer inserted into the center comes out without wet batter.
Make the Toffee Sauce
While the pudding bakes, combine the brown sugar, butter, heavy whipping cream, black treacle, vanilla, and salt in a medium saucepan.
Heat over medium-low heat, stirring until the butter melts and the sugar dissolves. Bring to a gentle simmer and cook for approximately 5 minutes, stirring occasionally, until smooth, glossy, and slightly thickened. The sauce will thicken further as it cools.
Assemble and Serve
Once baked, allow the sponge to rest for 2–5 minutes, then prick the surface all over with a skewer. Slowly pour approximately one-third of the warm toffee sauce over the sponge. Leave for 5–10 minutes to absorb.
Cut into eight portions and serve warm with the remaining toffee sauce and vanilla ice cream, cold pouring cream, or custard.
Notes
Dates: Leave the softened dates finely chopped for a little texture, or purée them with their soaking liquid for a smoother sponge. The finely chopped version was used in the original successful test.Black treacle: Unsulphured molasses can be substituted, although the flavor will differ slightly. Avoid blackstrap molasses.Baking dish: Use a 10×8-inch baking dish, at least 2 inches deep. Different pan sizes or materials may affect the baking time.Storage: Refrigerate leftover pudding and toffee sauce separately in airtight containers for up to 4 days. Reheat gently before serving.Lower elevations: I developed and tested this recipe at approximately 8,500 feet. For baking closer to sea level, try increasing the baking powder to 1½ tsp, reducing the milk to 75 ml (5 tbsp), and baking at 350°F (175°C). Begin checking for doneness around 35 minutes.