Heat the unsalted butter in a large skillet over medium-high heat until melted. Add the bread cubes to the skillet and toss to coat. Toast the bread, tossing every minute or so, until the bread cubes are golden-brown on all sides, about 5 to 7 minutes total. Remove the skillet from the heat. Sprinkle the chopped dill over the croutons and toss to coat them evenly. Serve immediately or let cool completely. Croutons can be made a few days in advance. Store leftover croutons in an airtight container at room temperature for up to 1 week.