Pour butter into an ungreased 9-in. square baking pan; sprinkle with brown sugar. Arrange peach slices in a single layer over brown sugar.
In a small bowl, cream shortening and sugar until light and fluffy. Beat in egg. Beat in lemon juice and vanilla. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk, beating well after each addition. Spoon over peaches.
Bake at 350° for 45-50 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before inverting onto a serving plate. Serve warm with whipped cream if desired.