Line 9×9 pan with foil and make sure you grease it very well.
Combine butter, cream, eggnog + sugar in heavy saucepan and bring to boil over medium heat, stirring occasionally.
Once boiling, reduce heat. Use a candy thermometer to reach 234°- soft boil stage.
****** Please note, if you have a thermometer the mix needs to reach 234 degrees, if not, as a guide, just boil for about 10 minutes, stirring and then if you fill a cup with cold water, drop a little bit of the mix into the cold water and if it's ready, it forms a soft ball.
Remove from heat and add white chocolate, stir until melted. Add marshmallow cream, nutmeg and vanilla. Stir until all combined and smooth. Add walnuts if using.
Pour into pan and garnish with a bit more nutmeg.
Allow to cool at room temp completely. Several hours at least. Overnight is better.
Cut into squares and store in an airtight container in the fridge.