In the bowl of a stand mixer, fitted with the paddle attachment, beat butter and sugars together until light and fluffy. Add egg and beat until well incorporated.
Add eggnog and vanilla and mix until combined.
In a separate bowl, combine flour, cornstarch, salt, baking powder, cinnamon and nutmeg.
Add the dry ingredients into the wet ingredients and pulse until combined. If the dough becomes to tough for the mixer, continue mixing the dough by hand until just combined. Divide the dough in half and refrigerate for at least one hour or overnight.
Preheat over to 350 degrees F.
On a lightly floured surface, roll out the dough until its 1/4 inch thick. ** I roll my dough out between two lightly floured pieces of parchment paper. I have very little counter space and this allows me to rotate the dough a lot easier or move it to another location if I need to. It's also a lot easier to clean up!
Cut cookies into whatever shape you desire. I used a 3-inch linzer cookie cutter to get the fluted edges.
Bake cookies for 8-10 minutes, or until the edges are just slightly turning brown. **I prefer just the bottoms to be slightly brown, so I found 9 minutes to be the magic time for my oven. It will also depend on how thin you make the cookies. If some are thinner, they will brown faster and become crispy.
Transfer cookies to a cooling rack.