Grease a 13- x 9- x 2-inch baking pan heavily with butter; set aside.
Cream butter; gradually add sugar, beating well. Add eggs, one at a time, beating well after each addition.
Combine flour, baking powder, and salt; add to creamed mixture alternately with milk, beginning and ending with flour mixture.
Pour batter into prepared pan and bake at 375° for 25 to 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes; remove from pan and allow to cool on wire racks.
Wrap cake tightly in foil; freeze for several hours or until firm. Slice cake horizontally and brush layers with Simple Syrup; spread strawberry jam on lower layer, and replace with top layer (layers will be thin).
Cut cake in 2-inch squares. Place squares two inches apart on a wire rack; place rack in a large shallow pan. Quickly pour the warm frosting over cakes, completely covering top and sides. Spoon up all frosting that drips through the rack, and reheat to pouring consistency; add a small amount of water, if necessary, to maintain original consistency. Continue pouring and reheating until all cakes are frosted. Allow frosting to dry.
Place cakes on a cutting board; using a sharp knife, trim any surplus frosting from the bottom of each cake.