Position one baking rack in the bottom third of the oven and one in the top third of the oven, then preheat the oven to 350°F/177°C.
Grease an 8-inch (20cm) x 8-inch (20cm) square cake pan with oil and line the bottom with parchment paper.
Make the date puree: In a saucepan, add 6 ounces pitted Medjool dates and 177 ml water, and bring to a boil, then turn the heat off. Cover and let sit for 10 minutes. Blend date-water mixture to a smooth paste and set aside to cool slightly so that the puree isn't hot enough to curdle the eggs.
Mix dry ingredients: In a medium bowl, combine 188 grams all-purpose flour, 1 teaspoon baking powder, 1 teaspoon baking soda, and ½ teaspoon kosher salt and whisk until combined.
Mix wet ingredients: In a stand mixer fitted with a paddle attachment, beat 56 grams unsalted butter and 144 grams dark brown sugar at medium speed until fluffy, about 3 to 5 minutes. Add in 1 large egg and 1 teaspoon vanilla extract and beat until fully incorporated and thick and ribbony (it may appear curdled; that's fine). Reduce speed to medium-low, and slowly add in the date puree and beat until combined.
Finish the batter: Reduce the speed to low, and slowly add in the dry ingredients. Batter will be thick, like buttercream frosting. Do not overmix.
Bake the cake: Pour into the cake pan and bake on the bottom-third rack for 25 to 30 minutes, until a toothpick comes out with just a few crumbs. Make the whipped cream and toffee sauce while the cake bakes (see below for instructions).Remove cake from the oven and immediately prick all over with a fork. Place on a wire rack to cool slightly until warm, but not hot.
Make the whipped cream:
Wipe the bowl of the stand mixer clean, and add 250 ml heavy cream, 16 grams powdered sugar, and 1 teaspoon vanilla extract. Using the whip attachment, whip at medium speed until mixture just begins to thicken (about 1 - 2 minutes). This prevents the cream from spraying everywhere. Note: Whipped cream whips better at colder temperatures. To achieve this, rinse the stand mixer bowl with very cold water and dry it before whipping.
Taste and adjust powdered sugar if needed. Then, increase the speed to medium-high and whip for an additional 1 to 2 minutes until stiff peaks form. Keep chilled in the fridge, covered, until ready to serve.
Prepare the toffee sauce and finish the cake:
In a small saucepan over low heat, add 113 grams unsalted butter and whisk in 144 grams dark brown sugar. Increase heat to medium, whisking frequently until butter melts. Once the mixture starts to bubble, add 120 ml heavy cream, ¼ teaspoon kosher salt, and ¼ teaspoon vanilla extract. Let sauce bubble and simmer for 3 minutes until thick enough to coat the back of a wooden spoon. The sauce will thicken as it cools. Remove from the heat and sprinkle with flaky salt if desired.
Turn the broiler on to 'HI'. Pour ¾ cup of the toffee sauce on top of the cake; reserve the remaining toffee sauce for pouring on top later. Broil on the top-third rack for anywhere from 1 - 3 minutes, until the toffee sauce begins to bubble and caramelize on top of the cake. Remove from the oven and cool slightly, about 5 minutes. You will want to serve the cake while it's still warm, but it should be cool enough to handle.
Just before serving, re-warm the reserved toffee sauce. Cut the cake in individual slices, then transfer to individual dessert plates and pour additional toffee sauce and flaky salt on top, if desired. Serve with whipped cream on the side (if you dollop with whipped cream on top, the warm cake may cause the cream to melt).
Notes
Equipment: The recipe calls for an 8-inch (20cm) by 8-inch (20cm) square tin or cake pan, but I’ve had success with a 9-inch cake pan as well (you’ll just have a thinner cake and a faster bake time). Make sure it's broiler safe!Controlling Sweetness: This is naturally a sweeter dessert, so I like to balance the sweetness with flaky salt. You can also omit the sugar in the whipped cream, if desired, or serve it with ice cream which often tastes less sweet because of the temperature.Make-Ahead Instructions:Bake the cake, make the toffee sauce, and broil it. After the cake cools, wrap it tightly in cling film and refrigerate for up to 2 days. The toffee sauce can be stored in an airtight container for a week.To reheat, remove the wrap, cover with foil, and heat at 350°F/177°C for 5 minutes. Uncover and continue heating until warmed through. Meanwhile, whip the cream. This method may result in a slightly denser, drier cake but remains delicious. It's also perfect for reheating leftover cake slices.