Make the cider syrup. Combine apple cider with maple syrup and brown sugar in a medium saucepot. Bring to a boil over medium-high heat. Reduce to low and continue to simmer for 30 minutes or until the syrup has reduced by half.
Meanwhile, make the whipped crème fraiche. Whip the heavy cream with powdered sugar until stiff peaks form. Fold in the crème fraiche. Chill until ready to serve.
Prepare the batter for the sweet potato french toast. Blend together mashed sweet potato with egg whites, eggs, vanilla, cinnamon, nutmeg, and a pinch of salt until smooth. The batter will be thicker than the normal french toast batter.
Dip the sliced bread into the batter, scraping off the excess. Melt 1 tablespoon butter in a griddle or non-stick pan over medium-low heat. Cook the french toast until browned, about 3 minutes. Flip and cook on the other side, another 2-3 minutes. Remove from heat, melt another tablespoon of butter, and repeat with the remaining bread.
Serve sweet potato french toast with cider syrup and whipped crème fraiche.
Notes
The cider syrup can be prepared up to 5 days in advance. The syrup thicken as it cools so if it gets too thick, reheat it on the stovetop or in the microwave until warm.