In a shallow bowl, whisk together the eggnog, eggs, vanilla, cinnamon, nutmeg, and (if using) pumpkin pie spice and rum extract until smooth.
Heat a large skillet or griddle over medium heat. Add a little butter and let it melt until it sizzles.
Dip each slice of bread into the eggnog mixture, coating both sides. Let excess drip off, then place the bread on the hot skillet.
Cook 2–3 minutes per side, flipping once, until golden brown and cooked through. Add more butter between batches as needed.
If cooking in batches, keep finished slices warm on a baking sheet in a 200°F (95°C) oven until ready to serve.
Serve warm with whipped cream, fresh berries, a dusting of powdered sugar, and syrup. Add toasted pecans and orange zest if desired.
Notes
Storage: Refrigerate leftovers in an airtight container for up to 3 days. Reheat in a 350°F (175°C) oven or toaster oven until warmed through (microwave works but softens the exterior).Make-ahead: Whisk the custard the night before and refrigerate. Stir before dipping bread.Tip: Day-old, thick bread holds up best and absorbs the custard without getting mushy.