In a large bowl, mix ricotta, 2 tablespoons sugar, and 1/2 teaspoon vanilla extract together. Divide the ricotta mixture among 5 slices of bread, then spread lightly almost to the edges of the bread. Top 3 ricotta-spread slices with the remaining 3 uncoated slices to make sandwiches.
In a medium-size bowl, mix milk, eggs, 1/4 teaspoon cinnamon, 1/2 cup sugar, and the remaining 1 teaspoon vanilla extract together to make a custard; set aside.
Submerge one of the sandwiches in the custard until it is soaked. Place in the bottom of a 4- to 6-quart oval slow cooker, setting it toward one end of the crockery insert. Submerge another sandwich in the custard and set it toward the other end of the insert. Place the 2 remaining slices of ricotta-spread bread, ricotta side down, on top of the sandwiches. There will be gaps to one side of each sandwich. Cut the remaining sandwich in half, submerge each half in the custard, and wedge each half into a gap. Pour the remaining custard over the top, then let the contents sit until the mixture is mostly absorbed, about 5 minutes.
In a small bowl, mix the remaining 2 tablespoons sugar and 1/4 teaspoon cinnamon together. Sprinkle over the top of the French toast, cover, and cook on low 4 to 5 hours. If your slow cooker has a warming setting, you can keep the French toast warm several hours; if it doesn’t, the French toast will stay servable up to 1 hour with the cooker turned off. Cut into 8 thick slices and serve with applesauce or fresh fruit, if desired.
