Place piece of wax paper on a baking sheet (should be at least 9x13 inches). Spread out the pretzels haphazardly in roughly the shape of a rectangle about 8x11 inches. Sprinkle almonds, if using. And toss sprinkles throughout, if using.
Prepare the fudgy bark: In a heavy bottomed large saucepan, add the eggnog and sugar. Bring to a rolling boil over medium to high heat (watch that the mixture does not boil over the pot- if boiling too hard, turn down the heat slightly, but continue a rapid boil), stirring constantly. Boil for 2 minutes exactly.
Remove from the heat and stir in the marshmallows, cinnamon and nutmeg. Put the saucepan back on the heat; bring back to a rolling boil for exactly 6 minutes, stirring constantly (turn down slightly if the mixture is bubbling over). The mixture will begin to turn a caramel color.
After 6 minutes of boiling, remove from heat. Immediately add the butter and white chocolate. Stir rapidly, until the mixture has a glossy appearance and the chips and butter are well mixed.
Pour the mixture over the pretzels, making sure you are moving the pan quickly while pouring to cover the whole rectangle. The mixture will begin to solidify fairly soon so you'll need to work quickly.
Let cool until completely room temperature. Lift up from the wax paper and break into chunks to serve.
Notes
Best if eaten within a couple of days. If you live in a damp climate, the pretzels might begin to soften after 24 hours or so.