Spread the pistachio mixture evenly on the phyllo in the baking dish. Top with the remaining phyllo sheets, buttering each sheet as above. Drizzle any remaining butter over the top. Using a sharp knife, make diagonal slices 1 1/2 inches apart all the way through the baklava; slice similarly in the opposite direction to make a diamond pattern. Bake for about 20 minutes, or until evenly browned on top.