In a shallow bowl, whisk together eggs, milk, brown sugar, cinnamon, vanilla, and salt until fully combined.
Preheat a medium skillet over medium heat and melt a small portion of the butter.
Add apple slices and sauté until slightly softened and caramelized, about 5 minutes; then set aside.
Dip each slice of bread into the egg mixture, letting it soak briefly without becoming soggy.
Cook the soaked bread in a buttered pan for 2–3 minutes per side, turning until golden brown and crisp at the edges.
Top the cooked toast with the warm apples and any pan juices; optionally sprinkle more cinnamon on top.
Serve immediately with maple syrup or a dusting of powdered sugar.