Whisk together vinegar and miso in a small bowl. Heat a large cast-iron skillet over medium. Generously coat radishes and skillet with cooking spray. Add radishes to skillet; cook, stirring occasionally, for 10 minutes or until just tender. Add butter, and sprinkle with sugar; cook 30 seconds, stirring to coat. Remove from heat; add miso mixture, and stir until radishes are completely coated. Add cilantro; toss to combine. Sprinkle with sesame seeds.