In a medium bowl, whisk the flour, baking powder, baking soda and 1/2 tsp. salt. Using an electric mixer, beat the brown sugar and 6 tbsp. butter on high until fluffy, scraping down the bowl. Beat in the eggs 1 at a time. On low speed, mix in half of the flour mixture. Mix in the date mixture, milk, vanilla and remaining flour mixture. Scrape into the pan, smoothing the top. Bake until the top is springy, about 50 minutes.