In a small bowl combine gelatin and water, let stand for 5 minutes.
In a medium saucepan combine cream, eggnog, sugar, and salt. Set over medium heat and stir occasionally until hot. Remove from heat. Stir in gelatin mixture. Cool slightly, about 10 minutes, stirring occasionally to prevent gelatin from sinking to the bottom. Remove to an airtight container and chill in fridge until set, or up to 3 days.
To make French toast:
In a large shallow bowl or dish whisk together eggs, milk, vanilla, salt, cinnamon, and allspice. Cut bread into 3-inch wide slices. With a paring knife, cut halfway into each slice to create a pocket for the filling. Fill each pocket with eggnog filling. Soak stuffed bread slices in egg mixture.
While bread is soaking, heat a large skillet or griddle on medium heat. Melt butter in pan and brown bread slices, working in batches if necessary. Brown 3-4 minutes per side, or until golden.
Notes
Note: You will probably have leftover eggnog filling, but don't worry. It's scrumptious.