Heat oven to 350°F. In large bowl, mix base ingredients with electric mixer on low speed until crumbly. With floured fingers, press mixture firmly in bottom of ungreased 13x9-inch pan. Bake 20 to 25 minutes or until light golden brown.
Meanwhile, beat eggs slightly in large bowl. Mix in granulated sugar, 1/4 cup flour and the baking powder until well blended. Stir in margarita mix and lime peel.
Remove pan from oven. Pour filling over warm base. Bake 18 to 22 minutes longer or until top is golden brown and filling is set. Cool completely, about 1 hour.
Just before serving, sprinkle bars with 1 tablespoon powdered sugar. Cut into 6 rows by 6 rows.