Preheat Oven & Prep Pan: Preheat your oven to 356ºF (180ºC) fan-forced. Grease a 11 x 7-inch (20cm x 30cm) brownie pan with butter and line it with parchment paper. For non-stick pans, lightly grease with butter, line the base with parchment and dust a light layer of flour on the sides. Set aside.
Prepare Dates: In a large mixing bowl, add chopped dates and cover them with the boiling water. Let them stand for 10 minutes to soften.
Mix Wet Ingredients: Whisk the sugar, melted butter and oil directly into the bowl with the softened dates and their soaking liquid, do not drain the soaking liquid.
Add Egg: Whisk the egg into the mixture.
Incorporate Dry Ingredients: Add the flour, baking powder and baking soda and whisk until incorporated, ensuring not to over-mix.
Transfer Batter: Pour the batter into a prepared baking pan, smoothing out the top.
Bake: Bake for 20-25 minutes or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool: Allow the slice to cool in the pan for 15 minutes. Then, transfer it to a wire rack to cool completely before adding the butterscotch sauce.
Make Butterscotch Sauce: In a medium saucepan, combine the cream, butter, and brown sugar. Bring to a gentle simmer over medium heat, then reduce to low and simmer for 5 minutes, stirring frequently. Remove from heat and stir in the salt.
Cool & Thicken the Sauce: Let the sauce cool in the saucepan for 10 minutes, then transfer to a heatproof jug. Place in the fridge for about 30 minutes, or until it thickens to a pourable consistency.
Pour Over the Slice: Pour the thickened sauce over the cooled slice and spread it evenly with an offset spatula or the back of a spoon. For easiest cleanup, place the slice on a wire rack with a tray underneath to catch any excess.
Set & Serve: Let the sauce set before slicing so it sits neatly on top. Cut into squares and enjoy!