Whisk together the flour, baking powder, and salt in a small bowl. In the bowl of a stand mixer fitted with the paddle attachment, beat the remaining 5 tablespoons of butter and dark brown sugar together on medium-low speed until just mixed, about 1 minute. Add the molasses and vanilla and beat just to combine. Scrape down the paddle and bowl using a rubber spatula. Continue to mix on medium low, add the egg, and beat just until blended. Add the date mixture and mix to combine. Slowly add the flour mixture, beating just until blended.