Preheat oven to 350°F.
Prepare 9 x 13" baking pan by spraying with cooking spray and/or lightly but thoroughly flouring the bottom (if needed – my pan doesn't need this).
In a medium bowl, whisk flour, baking powder, and salt until thoroughly combined.
In a second medium bowl, combine sugar and butter until butter is completely incorporated into the sugar. The mixture will be crumbly, similar to wet sand. (I find it's easiest to do this with my hands, rubbing the butter into the sugar with my fingers, but you can also use an electric mixer.) Add eggs, water, and vanilla, and stir until thoroughly combined. Add the dates and walnuts and stir again to distribute evenly.
Lightly dump the dry ingredients (flour mixture) over top of the wet ingredients (sugar mixture) and stir just until thoroughly combined. The dough will be very thick and sticky.
Using a rubber scraper, spread the dough into the pan, pushing it and flattening it until it's an even thickness (dough will only thinly cover the entire pan).
Bake for about 20-22 minutes, until lightly golden brown.
Let cool completely in the pan on a cooling rack before cutting. Cut into 39 sticks that are 1 inch wide and 3 inches long.
Place at least 1/4 cup of powdered sugar in a wide bowl. Roll each bar in the sugar to coat, tapping gently to remove any excess, loose sugar. (You should actually use only about 1 tablespoon of the sugar to thinly coat the bars, but it makes it much easier to work with them if there is plenty of extra sugar in the bowl. You can discard the excess.)
Freezing and make-ahead tips: These freeze really well, so they're a great make-ahead recipe for keeping some cookies in the freezer, ready in a pinch. You can cut the bars and roll each in powdered sugar prior to freezing (so you can pull them out individually as needed). Or, you can wrap and freeze the full pan, then defrost it, cut the bars, and roll them in sugar after freezing.