Heat oven to 350°F. Lightly coat a 10x15-inch jelly roll pan with cooking spray. In a medium bowl, whisk together flour and baking powder; set aside.
Using an electric mixer on medium speed, beat together sugar and butter until combined and crumbly. Add eggs, water, vanilla and salt; beat until thoroughly combined, scraping sides of bowl as necessary. Reduce speed to low; gradually add flour mixture. Using a rubber spatula, fold in dates and walnuts.
Spread batter in pan; bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool, cut into squares and dust with powdered sugar. Store in an airtight container or covered tightly with plastic wrap, at room temperature for up to 1 week. Serves 12.
Approximate Nutrient Analysis per bar:
430 calories, 10 g fat, 2.5 g saturated fat, 65 mg cholesterol, 400 mg sodium, 79 g carbohydrate, 4 g fiber, 51 g sugar, 8 g protein