Using a serrated knife, split the croissants horizontally.

Whisk together the milk, eggs, brown sugar, vanilla extract, ground cinnamon, and kosher salt in a wide, shallow dish until combined.

Melt 1 tablespoon of the unsalted butter in a large skillet over medium heat. Soak 2 of the croissant pieces in the egg mixture for 15 to 20 seconds per side, then transfer to a plate.

Once the butter is sizzling, add the soaked croissant pieces in a single layer to the skillet. Cook until golden-brown and crisp, about 2 to 2 1/2 minutes per side.

Repeat the soaking and cooking process with the remaining croissant pieces, using the remaining tablespoon of butter for the second batch.

Transfer the French toast to a serving plate. Dust with powdered sugar and serve with maple syrup, whipped cream, or fresh berries.