Preheat the oven to 350°F (175°C). Grease the baking pan and line it with parchment paper.
Place the chopped dates in a large heatproof bowl. Pour the boiling water over them, then stir in the baking soda. The mixture will foam. Let it sit for 10 minutes, then mash with a fork or whisk until mostly broken down. Small pieces of date are fine.
In a separate large bowl, beat the softened butter and brown sugar for 2 minutes, until light and fluffy.
Beat in the eggs one at a time, then the vanilla and molasses.
In a small bowl, whisk together the flour, baking powder, cinnamon, and salt.
Add half the dry ingredients to the butter mixture and fold gently. Fold in half the date mixture, then repeat with the remaining dry ingredients and dates. The batter will be fairly loose.
Pour the batter into the prepared pan and bake for 30-35 minutes, until the top springs back lightly and a toothpick inserted in the center comes out clean.
While the cake bakes, make the sauce. In a medium saucepan, melt the butter over medium heat. Stir in the brown sugar and cream, bring to a gentle simmer, and cook for 4-5 minutes, stirring often, until the sauce thickens slightly and coats the back of a spoon. Remove from heat and stir in the vanilla and sea salt.
As soon as the cake comes out of the oven, poke holes across the surface with a skewer. Pour about one-third of the warm sauce over the cake and let it soak in for 10 minutes.
Cut into 9 squares and serve warm with the remaining sauce spooned over the top.