Preheat the oven to 350F. Lightly grease 6 ramekins or a 9-inch, round soufflé dish.
Put the chopped dates into a bowl. Sprinkle them with the baking soda. Pour the boiling water over the dates and stir to make sure they're all covered. Allow the dates to soak for 20 minutes.
While the dates soak, whisk together the flours, baking powder, salt, and sugar in a large mixing bowl. When the dates are ready, add them—along with their soaking water—to the dry ingredients, followed by the melted vegan butter. Stir till the batter is evenly combined.
Divide the batter into the ramekins and place them onto a baking sheet. Transfer the sheet to the oven. Bake for 25 minutes, or until the tops of the puddings are domed and set. Allow the puddings to cool for 10 minutes before inverting them out of their ramekins.
To make the cashew sauce, combine all ingredients into a saucepan and bring to a simmer over low heat. Heat the sauce, stirring constantly, for 3-5 minutes, or until it's slightly thickened. Pour it over the puddings and serve them warm.