1 smallish baguette or crusty loaf of bread, cut into 1-inch slices (about 1 lb loaf)
FOR THE CUSTARD:
3 large eggs
1/2 cup milk
1/3 cup heavy cream
2 teaspoons pure vanilla extract
1/4 teaspoon ground cinnamon
pinch of sea salt
FOR THE SUGAR COATING:
1/2 cup (1 stick or 8 tablespoons) unsalted butter
3/4 cup packed dark brown sugar
2 tablespoons light corn syrup
1 teaspoon pure vanilla extract
1 Day Ahead:
Butter a 9×9-inch pan on the bottom and sides. Place the bread slices, standing up, in the pan. Set aside while preparing the custard.
In a large measuring cup or pitcher, whisk together the eggs, milk, cream, vanilla, cinnamon and salt until well combined. Pour the mixture evenly over all the bread slices in the pan. Cover the pan tightly with plastic wrap and refrigerate overnight.
Preheat the oven to 350 degrees F. Remove the french toast casserole from the refrigerator and set it on a baking sheet.
In a large microwave safe bowl or heat proof large glass measuring cup, melt the butter in the microwave till just melted (about 1 to 1 1/2 minutes on high). Add the brown sugar and corn syrup, whisk well to combine. Microwave on high again in 30-second bursts (two to three times should be enough) till the sugar is melted and the mixture is bubbly and hot. Gently whisk in the vanilla extract. (Alternatively, you can do this process in a pan on your stovetop.)
Carefully and evenly pour the sugar mixture over the french toast casserole, covering each slice well. Place the casserole, on the baking sheet, into the oven and bake until puffy all across the pan like a good bread pudding, and is golden and baked through, about 40 to 45 minutes depending on how packed your pan was. The casserole will deflate as it cools. Serve warm with mixed berries.