In a small bowl, sprinkle the powdered gelatin over ¼ cup of the milk and let stand until the gelatin is softened, about 5 minutes. In a medium saucepan, combine the remaining 2 ½ cups of milk with 2 tablespoons of the sugar and the salt and bring just to a simmer over moderately heat. Remove the milk from the heat and whisk in the softened gelatin until dissolved.

In a large microwave-safe bowl, melt the chopped chocolate at high power in 10-second intervals. Gradually whisk in the hot milk mixture until creamy. Strain the panna cotta mixture through a fine sieve into a large measuring cup.

Lightly brush six ½-cup ramekins with oil and set them on a baking sheet. Carefully fill the ramekins with the panna cotta mixture and refrigerate them for about 2 hours, until the panna cottas are firm.

Meanwhile, line a baking sheet with a silicone mat or lightly oiled parchment paper. In a small bowl, toss the pepitas with the cinnamon and nutmeg.

In a medium saucepan, combine the remaining ¾ cup of sugar with ⅓ cup of water and cook over high heat, swirling the pan gently, until a medium amber caramel forms, about 6 minutes. Remove from the heat and swirl in the butter and spiced pepitas. Pour the brittle onto the prepared baking sheet and spread it in a very thin layer. Let cool completely, about 20 minutes, then crack it into shards.

Run a knife around the panna cottas and invert onto plates. Garnish with the pepita shards and serve.