In a small saucepan, combine 1 1/2 sticks butter, dark brown sugar, vanilla extract, 3 tablespoons maple syrup, and ½ teaspoon salt. Bring to a boil over medium-high heat, whisking until smooth. Reduce heat to maintain a brisk simmer and continue cooking until thickened and slightly reduced, 3 minutes more. Transfer to an 8” x 8” baking dish and set aside to cool slightly.
In a large measuring cup, whisk to combine eggs, milk, cream, remaining 3 tablespoons maple syrup, ¼ teaspoon cinnamon, and ½ teaspoon salt. Place challah in two layers on top of syrup in baking dish, ripping pieces to fill spaces if necessary. Pour milk mixture over challah. Let sit one hour, flipping the top pieces of challah 2-3 times, to help custard saturate slices.
In a small bowl, combine 2 tablespoons sugar and remaining ¼ teaspoon cinnamon. Distribute remaining 3 tablespoons butter slices on top of bread and sprinkle cinnamon-sugar mixture on top. Place baking dish on a parchment-lined baking sheet and transfer to oven. Bake until puffed, golden, and a toothpick inserted in the middle comes out clean, about 45 minutes.
While French toast bakes, combine berries, lemon juice, and remaining 1 tablespoon sugar in a bowl. Toss to combine and set aside to macerate.
Remove french toast from oven and scoop, serving with pan syrup and macerated berries. If desired, serve with additional syrup and lightly whipped, unsweetened cream.