Spread butter on one side of each bread slice. Lay slices on a piece of waxed paper, butter side down. Spread 1 heaping tablespoonful of Nutella over four slices. Sprinkle chocolate over, leaving a 1/4-inch border. Cover with remaining bread, butter side up.
Warm a griddle or large nonstick skillet over medium-high heat. Working in batches, place panini on griddle and cook, pressing down with a spatula, until golden brown, about 2 minutes. Turn and cook on the other side until golden, about 2 minutes. Cool slightly and use a sharp cutter to form heart shapes, or cut diagonally into triangles. Repeat with remaining panini.