Slice the loaf of bread, if necessary; set aside to dry out a bit on the countertop; this can be for 15 minutes, as you prep the batter, or up to 48 hours.
Once ready to make the toast, place a metal wire rack in a rimmed baking sheet; place in an oven and pre-heat to 250°F; alternatively, use a warming oven on medium heat.
In a large bowl, combine the eggnog, rum (if using), eggs, cinnamon, nutmeg and vanilla.
Whisk until uniform in color.
Dunk two or three pieces of bread into the batter, flipping over to make sure both sides get covered.
In a large skillet or griddle, on medium heat, heat 1 tablespoon of butter.
Once the butter has melted, using tongs or your fingers, move the soaked bread from the bowl allowing excess batter drip off the bread; if you left the toast to soak overnight, be careful as the bread will be more fragile.
Fry the toast for 2 to 3 minutes or until the bread is golden brown.
Flip the toast and continue to cook for an additional 2 or 3 minutes until both sides are equally brown.
While the toast is cooking, place more slices in the bowl to soak up the batter.
Move the finished toast to the wire rack in the oven (or warming oven); repeat with the remaining bread, soaking first then frying.
Serve immediately with maple syrup or whipped cream.
