In a small saucepan, melt the 2 sticks (1 cup) of butter). Set it aside and let it cool.
Preheat the oven to 350 F. and lightly brush a small amount of the melted butter in a 9x13 inch pan.
Carefully unroll the phyllo dough and keep it covered with a damp kitchen towel to prevent it from drying out.
Take one or at most two sheets at a time and scrunch it lengthwise to create ruffles. Place the ruffled sheet into a 9x13-inch baking dish, starting at one end. Repeat with remaining phyllo sheets, filling the dish with evenly spaced ruffles.
Bake the plain ruffled phyllo in the preheated oven for 10 minutes to lightly crisp it.
Remove the baking dish from the oven and generously pour the melted butter over all the ruffles, ensuring it soaks into the layers.
Return to the oven and bake for another 10 minutes, until the phyllo is golden and crisp.
While the phyllo bakes, whisk together the eggnog, eggs, granulated sugar (if using), and vanilla extract in a medium bowl until smooth.
Carefully pour the eggnog custard evenly over the golden phyllo ruffles, ensuring the custard fills the crevices.
Return the dish to the oven and bake for 35–40 minutes, or until the custard is set and the top is golden brown.
Allow the pie to cool slightly to room temperature. Dust generously with powdered sugar and freshly grated nutmeg for garnish. Slice into squares and serve.