For white polenta, bring chicken stock to the simmer in a saucepan over medium heat. Add polenta in a thin steady stream, whisk continuously until all is incorporated, then stir occasionally until polenta is cooked through (15-20 minutes). Add parmesan and butter.
Meanwhile, preheat oven to 200C. Remove tenderloins from breasts and slice each breast lengthways into 5 pieces. Working with two pieces at a time (including tenderloins), pound with a meat mallet between 2 pieces of baking paper to 4mm thick.
Lay each slice on a work surface, top with a slice of prosciutto, then a slice of mozzarella in the centre. Season to taste, roll to enclose, place one sage leaf on top and skewer with a toothpick to secure.
Heat 20gm butter in a large non-stick frying pan over high heat until foaming, add half the involtini and cook, starting sage-side down, until golden, turning once (1-2 minutes on each side). Transfer to a small baking dish, wipe pan with absorbent paper and repeat with 20gm butter and remaining involtini, then pour pan juices over and roast until just cooked through (5-10 minutes).
Meanwhile, heat remaining butter in pan until foaming, add remaining sage, cook until sage is crisp and butter is nut brown (1-2 minutes). Serve involtini with polenta and a drizzling of sage butter.
Note White polenta is available from select delicatessens. If unavailable, substitute regular polenta.