Frozen Mississippi mud bars solve a summer problem that Mississippi mud pie never could. The pie is magnificent, but it belongs in the refrigerator. It requires a pie plate, a server, and eight people arriving in sequence. The bar format fixes all of that: same three layers (crunchy chocolate cookie crust, fudgy brownie center, chocolate pudding on top), built in a 9x13 pan, frozen solid, lifted out by parchment, and sliced into rectangles that hold their shape long enough to survive outdoor...