Place the water in a small dish. Sprinkle the gelatin on top and allow it to bloom for 5 minutes.
Add the nectarines to a food processor and pulse until pureed. Strain the puree through a fine mesh sieve to remove any lumps.
In a small saucepan over medium heat, add the coconut milk, nectarine puree and honey. Stir until simmering and the honey has dissolved. Reduce the heat to low, add the vanilla and bloomed gelatin and whisk for several minutes, until all the gelatin has dissolved.
Pour the panna cotta into four 6oz ramekins and place in the refrigerator for at least 4 hours.
While the panna cotta is chilling, roast the plums. Preheat the oven to 375 degrees fahrenheit.
Slice the plums in quarters, then halves again (eight pieces per plum).
Place the plums on a small, rimmed baking sheet and sprinkle with the maple sugar. Cook for 30-40 minutes. Remove from the oven and let cool.
Once cool, place the plums and juice in a glass storage container (I use this one) and refrigerate.
Once the panna cotta has set, serve with several roasted plum slices.