Grease your molds with butter and place them on a parchment-lined baking sheet. If you're making rustic pieces instead, line a baking dish or baking sheet with parchment paper.

Combine the heavy cream, brown sugar, confectioners' sugar, vanilla bean seeds, and salt in a large saucepan. The mixture will foam and nearly double in volume as it cooks.

Bring to a boil, stirring regularly with a heatproof spatula.

Reduce the heat and let the mixture gently simmer for about 20 minutes, or until it reaches 239°F (115°C) on a candy thermometer.

Immediately pour the hot mixture into the prepared molds or spread it about ¼ inch (½ cm) thick in the lined pan. Work quickly, as it begins to harden almost immediately.

Let the candy set for 30 minutes. If using molds, turn them over so the bottoms can dry completely before storing.