Mix the dates with ¾ cup boiling water in a heatproof bowl and set aside for 10 minutes to soften, then mash with a fork to form a paste.
Meanwhile, beat ½ cup butter and ½ cup sugar in a medium mixing bowl using an electric whisk on medium speed until light and fluffy, scraping down the sides with a spatula halfway through, around 5 minutes. Whisk in the eggs, then fold in the soaked dates. Add the flour, baking powder, pumpkin spice and 1 ½ teaspoons flaky salt to the bowl, then fold through until combined.
Heat the oven to 350 degrees.
Transfer the mixture to a high-sided 9-by-13-inch baking dish, using a spatula to spread into an even layer, and scatter the pecans, 3 tablespoons of the date molasses, plus the remaining ¾ cup sugar and ¼ cup butter on top. (If you’re preparing this in advance, you can refrigerate it at this stage, up to 8 hours in advance of baking.)
Pour the remaining 2 cups of boiling water over the cake mixture. Bake for 35 to 40 minutes, until the caramel is bubbling up the sides of the cake, and the cake is set in the center. Remove from the oven and let rest for 10 minutes before serving.
Marble the remaining 1 tablespoon date molasses through the sour cream, and serve alongside the pudding.
