Preheat the oven to 350 F, and position a rack in the center of the oven.
Spray a 9 or 10-inch baking pan with non-stick baking spray. (An 8-inch pan is too small for this recipe.)
In a bowl, combine the flour, sugar and spices.
Drizzle with the melted butter and use a fork or your fingers to work the butter in until the topping is moist and crumbly.
With an electric mixer, beat the sugar and butter for 2 minutes until light and fluffy, then beat in the eggs.
Add the eggnog, sour cream and vanilla extract, and use a whisk to whisk everything together until well combined.
In a separate bowl, sift together the flour, baking powder, baking soda and salt. Add the dry ingredients to the liquid ingredients, and whisk just until moistened.
Spread the cake batter into the prepared pan, then sprinkle with the crumb topping.
Bake the cake for about 35-40 minutes, until a cake tester or toothpick inserted in the center comes out clean.
Let the cake cool for 20-30 minutes before serving.
As the cake is cooling, you can mix up the icing.
In a bowl, whisk together the powdered sugar and eggnog until smooth.
Drizzle the icing over the cake. The icing will set in a few minutes.
After the cake cools completely, you can cover it tightly with plastic wrap, or store it in an airtight container at room temperature for up to 5 days.Rewarm pieces of cake in the microwave for 30-60 seconds at 50% power, for a soft, just-baked texture.