Heat oven to 350°F (175°C). Grease an 8-inch square pan or a similar baking dish. Line the base with parchment for easy removal.
Soften the dates: Combine chopped dates and boiling water in a bowl.
Stir in baking soda and let sit 10 minutes. The mixture will soften and thicken.
Blend or mash: Puree the date mixture with an immersion blender until mostly smooth (some small bits are fine). Alternatively, mash thoroughly with a fork.
Cream butter and sugar: In a mixing bowl, beat butter and brown sugar until light and fluffy, about 2 minutes.
Add eggs and vanilla: Beat in eggs one at a time, then add vanilla.
Scrape the bowl as needed.
Combine dry ingredients: In a separate bowl, whisk flour, baking powder, salt, and cinnamon if using.
Make the batter: On low speed, mix dry ingredients into the butter mixture just until combined. Fold in the date puree. The batter will be thick.
Bake: Spread into the prepared pan and bake 28–35 minutes, until the center is set and a toothpick comes out with a few moist crumbs.
Make the sauce: While the cake bakes, melt butter in a saucepan.
Add brown sugar and cream. Bring to a gentle simmer for 3–4 minutes, stirring, until smooth and slightly thickened. Remove from heat, stir in vanilla and a pinch of salt.
Soak and serve: Poke the warm cake all over with a skewer.
Pour about half the toffee sauce over the top and let it soak for 10 minutes. Serve warm slices with extra sauce and ice cream or custard.