Combine the sugar, butter, eggnog, nutmeg, and cinnamon in a large saucepan. Bring to a boil, stirring occasionally to melt the butter. Once the mixture reaches a rolling boil, stop stirring, and clip a candy thermometer onto the pan.
Heat mixture to 235 degrees F (113 degrees C), or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface.
Remove the pan from the heat and stir in the white chocolate pieces, marshmallow cream, vanilla, and walnuts. Beat the mixture with a wooden spoon until fluffy and it starts to lose its gloss. Spoon into the prepared pan, spreading evenly. Cool completely, then cut into small squares for serving.