SCRAPE any frosting off the leftover cake scraps. Be thorough. If you like, thinly slice off the edge where the icing was.
SLICE the cake as evenly as possible.
CUT the slices into cubes.
LAY the cubes in a single layer on a baking sheet.
DRY the chocolate cake cubes out overnight (at the minimum), but 24 to 48 hours is even better. You are trying to get pieces as dry as possible. (See Note #1 if you have fresh cake.)
PREHEAT the oven to 250°F (120°C). Line a baking sheet with parchment paper.
MELT the butter in the microwave for about 20 seconds.
POUR the cake cubes into a large mixing bowl.
STIR in the butter and granulated sugar. Mix until the cake cubes are coated.
LAY the coated cake cubes on the lined baking sheet.
TOAST the cake cubes in the oven for 10 minutes. Stir the cubes and return them to the oven for another 10 minutes. Repeat one or two more times. The total toasting time should be between 30 and 40 minutes. Hint: Keep baking until the cake cubes begin to look toasty and feel firm to the touch.
COOL the croutons completely. As they cool, they get crunchy. Serve them on top of ice cream or store them in an airtight container at room temperature.