Cut the bread into 1" cubes and pack into a 13x9" pan.
Whisk together the 6 eggs, 2 cups whole milk, ¼ cup brown sugar, 1 ½ tsp vanilla extract, 1 tsp ground cinnamon, and 1 tsp kosher salt.
Pour the custard mixture over the bread.
Melt 4 Tbsp butter and stir in ½ cup brown sugar, ½ tsp cinnamon, and a pinch of salt. Sprinkle this topping over the casserole.
Bake at 400°F for 30 minutes until golden brown.
Dust with powdered sugar and drizzle with syrup or your favorite garnish.
Serve warm.
