Cream butter in a large mixing bowl; gradually add 1 cup sugar, beating well. Add 1 egg and egg yolk; beat well. Stir in vanilla.
Combine flour, baking powder, and salt in a small mixing bowl; add to creamed mixture, stirring well. Press into a greased 13- x 9- x 2-inch baking pan; sprinkle with chopped pecans.
Beat egg white (at room temperature) in a small mixing bowl until foamy. Gradually add brown sugar, 1 tablespoon at a time, beating until stiff peaks form and sugar dissolves. (Do not under beat mixture.) Spread evenly over pecans.
Bake at 350° for 30 minutes or until lightly browned. Cool completely in pan. Cut into 2- inch squares.