In a medium bowl, beat the eggs together well. Beat in the milk, sugar and vanilla.
Layer the challah bread in a single layer across a large baking dish (I use my 9-inch x 13-inch baking pan). Gently pour the egg mixture over the bread slices. Allow to soak for 2 minutes. Gently flip using a spatula. Allow to soak an additional 2 minutes. Tip the pan around if needed to help the bread absorb the egg mixture. The bread should soak up all of the egg mixture.
Heat a large skillet on medium to medium-high heat. Melt a knob of butter on it. Gently transfer all 4 challah french toast to the pan, letting excess custard drip off each slice. Cook until the bottom is golden brown, about 3 minutes. Flip and cook until the other side is also golden brown, about 3 minutes. If you feel like your french toast is browning too quickly, turn the heat down.
Serve immediately. Top with butter, maple syrup and fresh berries, or use your favorite toppings.