Preheat oven to 425°.
1. Cut pastry into six 4" squares, place on a parchment-lined baking sheet, and prick all over with a fork. Top with plums, leaving a ½" border. Sprinkle with sugar; season with a few grinds of pepper.
2. Bake tarts, rotating pan halfway through, until edges of pastry are puffed and golden brown, 25-30 minutes.
3. Drizzle with honey and sprinkle with salt just before serving. (Tarts can be baked 4 hours ahead. Keep at room temperature.)