Brush eight 4-ounce ramekins with oil. Line a rimmed baking sheet with foil.
Sprinkle the gelatin over 1/4 cup water in a shallow bowl and let stand until softened, about 2 minutes.
Heat the coconut milk, heavy cream, 1/2 cup of the sugar and a pinch of salt in a small saucepan until hot but not simmering. Stir in the gelatin until dissolved. Divide the panna cotta among the prepared ramekins. Cover and refrigerate until set, at least 4 hours.
Meanwhile, mix the remaining 1/3 cup sugar with 3 to 4 teaspoons water (until the consistency of wet sand) in a heavy skillet. Set over medium heat and cook, swirling the skillet occasionally for even browning (do not stir the mixture), about 10 minutes. When the sugar is a deep caramel color, remove from the heat, stir in the peanuts and immediately pour onto the prepared baking sheet. Let cool completely.
When ready to serve, run a sharp knife around the edge of the ramekins, then invert the panna cottas onto serving plates. Coarsely chop the peanut brittle and sprinkle it on top of the panna cottas.
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