To make the cake, stir the dates, baking soda, and boiling water together in a large heat-proof bowl.
Let stand for 30 minutes, or until cooled to room temperature.
Toward the end of the cooling time, preheat the oven to 350F.
Grease and flour a 9x9-inch metal baking dish. Set aside.
Add the brown sugar, eggs, butter, and vanilla to the date mixture.
Mix on medium for 2-3 minutes. The batter may look curdled.
Add the flour, baking powder, and salt.
Mix in on low for 1 minute, or just until combined, scraping the sides as needed.
Pour the batter into the prepared baking dish.
Bake for 35-45 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs. You don't want wet batter. There should just be a couple moist crumbs.
Set the cake in the pan on a wire cooling rack while you make the sauce.
To make the toffee sauce, add the brown sugar, heavy whipping cream, butter, and salt to a medium saucepan.
Bring to a boil over medium heat, stirring regularly.
Reduce the heat to low and simmer for 2 minutes, or until the sugar has completely dissolved.
Remove from heat.
Whisk in the vanilla.
Let the sauce stand for 5-10 minutes to thicken before using.
Optional: Poke small holes over the top of the cake. I like to use a bamboo skewer.
Pour 1/2 cup of the sauce over the top of the warm cake.
Let the cake cool in the pan for about 10 minutes.
Serve the warm cake topped with the remaining sauce.
Notes
Dates - Mejool dates are usually recommended for use in this cake. I've also used deglet noor dates, and they worked out well.
Chopping the dates: Since dates are sticky, they can be difficult to chop. You can give the blade of your knife a quick spritz of nonstick cooking spray to help with that.
Brown sugar: You can use light or dark brown sugar in this recipe. Dark brown sugar will give the cake and the sauce a richer toffee flavor.
Butter: I prefer using salted butter in this recipe. However, you can use unsalted if you like.