Step 1: In a small saucepan, bring the apple cider to a boil over medium-high heat. Reduce the heat and simmer until the liquid is reduced to 1 cup, about 10-12 minutes. Remove from heat and let cool for 5 minutes.
Step 2: Pour 1/4 cup of the reduced cider into a small bowl and sprinkle the gelatin over it. Let it stand for 5 minutes to bloom.
Step 3: In a medium saucepan, combine the heavy cream, sugar, cinnamon, nutmeg, and salt. Heat over medium heat, stirring constantly, until the sugar is dissolved and the mixture just begins to simmer. Do not let it boil.
Step 4: Remove the cream mixture from the heat and whisk in the gelatin-cider mixture and the vanilla extract until the gelatin is completely dissolved.
Step 5: Stir in the remaining 3/4 cup of reduced apple cider. Pour the mixture through a fine-mesh sieve into 6 ramekins or dessert glasses.
Step 6: Refrigerate for at least 4 hours, or until completely set. Serve chilled with optional caramel sauce or apple slices.