Sift flour with baking powder and salt; set aside.
In large bowl, with wooden spoon, or portable electric mixer at medium speed, beat butter, sugar, eggs, and vanilla until light and fluffy.
Gradually stir in flour mixture until smooth and well combined.
Using rubber scraper, form dough into a ball.
Wrap in waxed paper or foil; refrigerate several hours, or overnight.
Divide dough into 4 parts; refrigerate until ready to use.
Meanwhile, make Filling: In small saucepan, combine dates and sugar with 1/2 cup water.
Cook, stirring and over medium heat, until mixture has thickened—about 5 minutes.
Remove from heat.
Stir in lemon peel, lemon juice, and walnuts.
Cool completely.
Preheat oven to 375F.
Lightly grease cookie sheets.
On lightly floured surface, roll dough, one part at a time, 1/8 inch thick.
With floured, 2-1/2-inch, round or scalloped cookie cutter, cut out cookies.
Reroll trimmings, and cut.
Using spatula, place half the cookies, 2 inches apart, on cookie sheets.
Spread 1 teaspoon filling over each cookie; cover with another cookie.
With floured fork, seal edges firmly; also prick center of top.
Bake 10 to 12 minutes, or until lightly browned.
Remove to wire rack; cool.
Makes 3 dozen.