Preheat the oven (not convection) to 350 degrees F. Coat 4-ounce aluminum tin molds with cooking spray and set aside.
Pour the hot water into a bowl with the dates and let soak for about 10 minutes. Mix in the baking soda. (Be sure not to let the dates soak too long or they will become dry.)
Meanwhile, combine the sugar and butter, then add the vanilla (it will look sort of broken). Add the eggs, one at a time. Mix in the flour. Pour the batter into the bowl with the dates and whip by hand to combine. The batter is very loose.
Pour the batter into the molds, filling them no more than halfway up. Bake for 15 to 20 minutes.
Immediately hot from the oven, cut off the uneven tops and dunk 6 to 7 at a time in the hot Toffee Sauce for a couple of minutes. Remove with a slotted spoon to a wire rack set over a sheet pan. Repeat until all the cakes have been soaked. Refrigerate before handling to allow the sauce to set in without breaking the cakes.
In a saucepan, melt the sugars and treacle with the butter until well incorporated, but don't allow the mixture to burn. Add a small amount of cream to facilitate dissolving the sugar. When the sugar mixture is smooth, whisk in the remaining cream slowly to prevent seizing. Bring to a boil to thicken, then remove from the heat and add the rum, if using. Keep the sauce covered until ready to use.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.