Line a large rimmed baking sheet with parchment paper or a silicone baking mat.

In a medium microwave-safe bowl, microwave half of the dark chocolate on HIGH in 30 second intervals until it begins to melt. Stir until smooth. Add the remaining half of the chocolate and stir until completely melted. If needed, return to the microwave for an additional 30 seconds.

Pour the dark chocolate in a puddle in the center of the prepared baking sheet and spread it into a large rectangle (roughly 8-by-12-inches). Let the chocolate set for 5 minutes, then sprinkle all over with the crushed freeze-dried raspberries. Sprinkle with edible rose petals and flaky sea salt. Let sit at room temperature until set, about 1 hour. Alternatively transfer to the freezer or refrigerator to harden, about 20 minutes. Break into pieces and serve. Store any extra pieces in an airtight container in the refrigerator.