Place piece of wax paper on a baking sheet (should be at least 9 x 13 inches). Spread out the pretzels haphazardly in roughly the shape of a rectangle about 8 x 11 inches. Sprinkle almonds, if using. And toss sprinkles throughout, if using.: As you lay the wax paper onto the baking sheet, notice the faint crinkle and the way the light dances off the smooth surface, which makes cleanup effortless later. Spread the pretzels haphazardly in a roughly rectangular shape about 8 x 11 inches so the sweet mixture can evenly coat them; if you make the rectangle too small the mixture will pool deeply, and too large will leave exposed gaps. When sprinkling the optional almonds and decorative sprinkles, distribute them so each section gets textural interest. A common mistake is crowding the pretzels tightly together, which prevents the bark from forming distinct shards, so keep small spaces for the coating to flow between pieces.
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.: When you add the eggnog and granulated white sugar to a heavy bottomed large saucepan, the mixture will look pale and loose at first, then begin to steam and thicken as it comes to a rolling boil. Stirring constantly prevents hot spots and gives you a smooth base, and during the boil you should hear a steady, confident bubbling rather than frantic splashes; this sound tells you the sugars are cooking properly. Boil for exactly 2 minutes, watching closely for boil overs; if the mixture threatens to spill, reduce the heat slightly but maintain a rapid boil. The visual cue to watch for is the surface moving vigorously with even bubbles. A typical misstep here is stepping away, which can lead to scorching or uncontrolled boil overs, so stay attentive.
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.: Off the flame, add the mini marshmallows , ground cinnamon , and freshly grated nutmeg , then return the saucepan to the heat. As it heats again, you will see the marshmallows soften, swell, and begin to melt into a silkier mass, and the aroma of spices will lift, signaling progress. Bring back to a rolling boil for exactly 6 minutes while stirring constantly; during this stage the mixture will deepen toward a caramel color, a visual confirmation that the sugars are developing flavor. The continuous stirring creates an even texture and prevents the mixture from sticking to the pan. Avoid undercooking here, which leads to a too soft bark, and avoid boiling too vigorously, which can cause the mixture to scorch or bubble over.
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.: Immediately incorporating the butter and white chocolate chips off the heat is essential because residual heat will melt them into a glossy emulsion without overheating the chocolate. Stir rapidly until the mixture turns glossy and the chips and butter are fully incorporated, which produces a shiny surface and a velvety texture you can almost see. The smell will shift from sweet caramel to a richer, buttery note. A misstep to avoid is lingering on the heat while adding chocolate, which can cause the white chocolate to seize into grainy clumps rather than forming a smooth coating.
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.: When you pour, keep the pan moving to achieve an even spread; the molten mixture will begin to set quickly, so work with a steady hand and pour in a slow, continuous stream to coat the pretzels and embedded almonds and sprinkles. You'll notice the mixture thicken visually as it cools, changing from glossy and fluid to denser and more satiny. Use a spatula to coax any drips into open spots, but do this quickly because it firms up fast. A common mistake is waiting too long to pour, which results in an uneven finish and harder to adhere pockets.
Let cool until completely room temperature. Lift up from the wax paper and break into chunks to serve.: Cooling is where the magic sets, transforming the shiny molten sheet into brittle, breakable bark. The texture firming is tactile when you lightly tap the surface and feel it resist pressure. Once fully cooled, lift the wax paper and break into chunks; the edges will be irregular and the cross sections reveal pockets of melted white chocolate and marshmallow. Store in a cool, dry place for best texture. Avoid refrigerating in damp climates, which can soften the pretzels prematurely; instead, keep in an airtight container at room temperature for a couple of days.