Coat an 8-inch square baking pan with cooking spray; line with aluminum foil, letting ends of foil extend over sides of pan. Coat foil with cooking spray; set aside.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, baking powder, and salt in a large bowl; stir with a whisk. Set aside.
Beat egg, sugar, butter, and vanilla with a mixer at medium speed until smooth and creamy. Gradually add flour mixture, beating well. Stir in dates and walnuts. Spread mixture into prepared pan. Bake at 350° for 22 minutes or until golden. Cool completely on a wire rack.
Remove brownies from pan using foil; cut into squares. Remove from foil using a spatula.