In a large bowl, whisk the eggs, heavy cream, milk, 1/2 cup sugar, vanilla, and cinnamon until smooth and homogenous.

Grease a baking dish well with butter so the slices do not stick.

Briefly dip each slice of brioche or challah into the custard. Let excess drip back into the bowl. Arrange the soaked slices in the buttered dish in a single layer or slightly overlapping.

Pour any remaining custard over the bread so the pieces are evenly moistened.

Cover the dish tightly and refrigerate for at least 2 hours or overnight.

When ready to bake, preheat the oven to 350°F (175°C). Remove the dish from the fridge and let sit a few minutes while the oven heats.

Bake uncovered for 30 to 35 minutes, or until the top is golden and the center is set when gently nudged.

Let the French toast cool slightly. Sprinkle an even layer of granulated sugar across the top.

Using a kitchen torch, caramelize the sugar until it forms a golden, crisp crust.