In a small bowl, stir together 8 oz mascarpone, 1/2 cup powdered sugar, and 1 tsp vanilla extract until smooth and easily spreadable.

Cut the challah or brioche into roughly 1-inch cubes and place them in a greased 9x13 dish, distributing evenly.

Dollop the mascarpone over the bread cubes and use a spatula to swirl slightly.

In a large bowl, whisk together 8 large eggs, 2 cups whole milk, 1 cup heavy cream, 3/4 cup granulated sugar, 1/4 cup brown sugar, 2 tsp vanilla extract, 1/2 tsp ground cinnamon, and 1/4 tsp salt until combined and slightly frothy.

Pour the custard over the bread and mascarpone in an even stream.

Scatter 2 cups sliced fresh strawberries evenly over the soaked bread and press down gently.

Cover tightly with plastic wrap or foil and refrigerate for at least 4 hours or overnight.

Preheat the oven to 350F and remove the casserole from the fridge, letting it sit for 20 minutes to take the chill off.

Dot the top with 2 tbsp butter pieces, cover with foil, and bake for 35 minutes.

Remove the foil and bake uncovered for an additional 10 minutes until the top is golden and a knife inserted near the center comes out mostly clean.

Let the casserole rest for 10 minutes before serving.

Dust lightly with powdered sugar if desired and serve with warm maple syrup.