In a shallow bowl or pie plate, whisk the egg, milk, vanilla, cinnamon, and salt together until fully combined.
Lay the slice of bread in the egg mixture. Let it soak for about 20-30 seconds on each side, making sure it's saturated but not falling apart.
Melt the butter in a small skillet over medium heat until it's sizzling.
Carefully transfer the soaked bread to the hot pan.
Cook for 2-3 minutes on the first side, until it's deep golden-brown. Flip carefully and cook for another 2-3 minutes on the other side.
Slide onto a plate and serve immediately.
