Mix cake according to manufacturer’s instructions. Deposit 3 pounds into greased half sheet trays with collars. Bake at 350F until cake is golden brown and springs back lightly when touched. Let cool completely.
Split the sheet cake horizontally, being careful to cut the layer perfectly even. Cut out 2-inch circles from the top and bottom layers, reserving cake scraps for another use (like cake pops). Pipe a dollop of filling on the top of a “bottom” circle, top with a “top” circle and decorate with fresh raspberries and mint leaves.