Using the top of an 8-ounce ramekin, cut out a circle from one slice of brioche. Repeat another 7 times. These will be the tops and bottoms of your charlottes. Slice the remaining 12 slices in half. These will be formed into the sides of the charlotte.
Preheat oven to 375 degrees. Grease and sugar 4 8-ounce ramekins. You will use 2 tablespoons of sugar for this. Place ramekins on a baking sheet and set aside.
Make the batter by whisking eggs in a medium bowl with milk, last 2 tablespoons of sugar, salt, and cinnamon. Grab the baking sheet with the ramekins. Dip one circle into the batter quickly and let the excess drip off. Place into the bottom of one greased and sugared ramekin. Repeat with the other 3 ramekins. Next, take one rectangle of brioche and dip into the batter, letting the excess drip off. Lay the rectangle along the side of the ramekin to form the walls. Repeat with 1 or 2 more slices, so that the wall is complete. Repeat with the other 3 ramekins.
Fill each brioche vessel with about ⅔ cup of cooked apples, making sure to fill to the top. Dip remaining circles of brioche into batter, letting the excess drip off. Place on top of the apples and firm press down to seal them inside. Top brioche with more sugar.
Bake charlottes in the center of the oven until the tops are toasted, golden brown, and the apple juices are bubbling around the sides, about 30 minutes. Remove from oven and let cool for 10 minutes. Slice a knife around the sides to release the charlotte and then flip onto a plate. Serve with cinnamon sabayon (recipe below).