Preheat the oven to 350 degrees Fahrenheit. Grease a 9-13” glass baking dish.
Put the 2 cups of pitted dates into a heat-safe container and pour 2 cups of boiling water over them. Allow to soak for 5 minutes.
Transfer the soaked dates into a food processor along with 1 cup of the soaking water (or 3/4 cup, if you want to use rum in your cake). Process until fairly smooth.
Add the 2/3 cup of rum (optional), the 2 tablespoons of molasses, the 4 eggs, and the 1 tablespoon of vanilla extract, and pulse a few times to mix. Add the chunks of the 6 tablespoons of unsalted butter gradually, pulsing each time.
Separately, mix together the 2 teaspoons of baking soda, the 1/2 teaspoon of salt, the 1/2 cup of demerara sugar, the 1/4 cup of granulated sugar, and the 1 1/2 cups of flour. Add gradually to the food processor, pulsing as you add it, until you have a well-mixed batter.
Pour the batter into your greased baking dish. (If you have a cake strip that fits, use it; the cake will be unevenly puffy in the center otherwise. If you don’t have one, don’t worry!)
Bake at 350 degrees Fahrenheit until a toothpick inserted near the center comes out mostly clean, around 25 minutes. (Be careful not to over-bake!)