Spread softened butter evenly around your baking dish and layer in your slices of brioche so that they over lap eachother.
In a large mixing bowl whisk together your eggs, eggnog, salt, nutmeg, cinnamon, granulated sugar, and vanilla extract. When well combined pour mixture over the layered brioche.
Preheat the oven to 375F. Sprinkle the brown sugar evenly over ther french toast. Bake for 40-50 minutes, until golden brown and crisp on the outside, then soft on the inside.
When golden and brown remove from the oven and sprinkle powdered sugar on top and serve and enjoy.