Beat brown sugar and mayonnaise at medium speed with an electric mixer until creamy. Add egg and vanilla, beating until blended.
Stir together flour, baking powder, and salt; gradually add to mayonnaise mixture, beating until blended.
Press dough into an ungreased 15- x 10-inch jelly-roll pan.
Bake at 350° for 15 to 17 minutes or until golden. Place pan on a wire rack. Sprinkle chocolate morsels over warm cookie. Let stand 5 minutes; spread chocolate evenly over top. Sprinkle with almonds. Cool in pan on a wire rack 1 1/2 to 2 hours or until completely cool. Cut into squares.