Preheat your oven to 350°F (175°C). Grease an 8×8‑inch square baking pan or similar 2‑quart baking dish with butter, then line the bottom with parchment paper. Lightly grease the parchment as well.

Place the chopped pitted dates in a heatproof bowl. Pour the boiling water over them, then stir in the baking soda. It will foam slightly. Let the mixture sit for 10–15 minutes to soften the dates.

In a medium bowl, whisk together the flour, baking powder, fine sea salt, and ground cinnamon (if using) until well combined and free of lumps.

In a large mixing bowl, beat the softened butter and brown sugar together with a hand mixer on medium speed for about 3 minutes, until light and fluffy. Scrape down the sides of the bowl as needed.

Beat in the eggs one at a time, mixing well after each addition. Add the vanilla extract and mix again until incorporated. The mixture may look slightly curdled; this is fine.

Using a fork or immersion blender, mash or lightly blend the softened dates and their soaking liquid until mostly smooth with a few small pieces remaining. Add this date mixture to the butter-sugar-egg mixture and stir until combined.

Add the dry ingredients in two additions, alternating with the milk. Stir in half of the flour mixture, then half of the milk, followed by the remaining flour and remaining milk. Mix gently after each addition just until incorporated, avoiding overmixing.

Spread the batter evenly into the prepared pan. Bake for 28–35 minutes, or until the top is golden brown and a toothpick inserted in the center comes out with moist crumbs but no wet batter. Begin checking around 25 minutes.

While the cake bakes, make the toffee sauce. In a medium saucepan over medium heat, combine the brown sugar, butter, and heavy cream. Stir as the butter melts and the mixture comes to a gentle simmer. Cook, stirring often, for 4–6 minutes until slightly thickened and glossy. Remove from heat and stir in the vanilla extract and a pinch of fine sea salt. Let the sauce sit for a few minutes; it will thicken further as it cools.

When the sponge comes out of the oven, let it rest for 5 minutes. Using a skewer or fork, poke holes all over the surface of the warm cake. Pour about half of the warm toffee sauce evenly over the top, allowing it to soak into the holes.

Let the soaked pudding sit for at least 10 minutes to absorb the sauce. Cut into squares or spoon into bowls. Serve warm, topped with additional warmed toffee sauce and, if desired, vanilla ice cream, whipped cream, and a few flakes of sea salt.