Place praline nuts and cocoa in blender, blend until finely ground. Working near a freezer, use a melon scoop to form four small scoops of vanilla gelato. Place the gelato scoops on a tray and place in freezer. Wait at least one hour. Next, take a large scoop of chocolate gelato. Still in the scoop, press your thumb into the middle of the gelato forming a cavity. Remove one small vanilla scoop from the freezer, and place into the chocolate gelato. Quickly shape the excess chocolate around the top of the vanilla. Release the tartufo onto a baking sheet, and cover with cocoa praline blend. Return to freezer immediately. Repeat this individual process three more times. Serve firm.