Preheat oven to 350º F. Grease or line an 8 inch baking pan with parchment paper.
In a bowl mix together all of the ingredients for the crumb topping. Use your fingers to mix well, it will be like coarse sand. Set aside.
Mix together the sugar and melted butter until smooth.
Add eggnog, sour cream, egg, and vanilla. Mix until everything is well combined and egg is fully incorporated.
In a separate bowl whisk together flour, baking powder, baking soda, salt and nutmeg until combined. Add to the eggnog mixture and mix until it just comes together.
Pour batter (it will be thick) into prepared baking dish. Sprinkle the crumb topping over, and gently press down.
Bake for 30-35 minutes, until a toothpick inserted comes out clean.