Scrape the butter into a medium bowl. Slice the vanilla bean in half lengthwise with a paring knife, then use the back of the knife to scrape out the caviar (a.k.a. the seeds) from the vanilla bean. Scrape the beans onto the soft butter and add the sugar and a large pinch of salt. Stir well to combine. Serving option 1 (more casual): Cover the vanilla butter in a small serving bowl and chill until you’re ready to serve, then take out of the fridge about 20 minutes before serving, so the butter is still cold but spreadable. Scrub the radishes and wash the leaves well. Chill well in the fridge or a bowl of ice water. Serve the radishes, tops and all, with the small bowl of vanilla butter and another small dish of flaky salt. Dip the radishes in butter and sprinkle with salt as you go (or lightly dip the buttered radish into the dish of salt). Serving option 2 (fancier): Form the vanilla butter into a log to chill: Lay out a piece of plastic wrap, scrape the butter onto it in a long, log-ish shape, then wrap tightly and roll until you have a round tube an inch or two in diameter. Chill until very firm, about an hour. Scrub your radishes and thinly slice them on a mandoline. Keep in a bowl of ice water if making ahead, then briefly drain on a clean kitchen towel before serving. When ready to serve, slice the butter into thin rounds and lay on crackers, then top with a few slices of radish and a sprinkle of flaky salt. Serve while still cold. Save any extra vanilla butter for toast or smearing on sandwiches tomorrow.