For the cookies:
For the filling:
Preheat the oven to 350°F. Line two cookie trays with parchment and set aside.
In a medium bowl, whisk together the flour, baking soda, cinnamon, nutmeg, ginger, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until it is smooth. Add the sugar and cream until light and fluffy. Add the egg, egg yolk, vanilla extract, and Eagle Brand® Sweetened Condensed Milk, and beat on medium-high until fully incorporated.
With the mixer on low, gradually pour the flour mixture into the butter mixture and beat until incorporated. With a spoon or small cookie scoop, scoop the dough into 1-teaspoon-sized balls, roll in granulated sugar (optional), and place on the prepared baking sheets about 3 inches apart. Bake until edges are golden, about 12–14 minutes. Remove from the oven and let cool on the trays for a couple minutes before transferring to a rack to cool completely.
Meanwhile, prepare the filling. In a medium bowl using a hand mixer (or in your stand mixer with the whisk attachment), beat together the butter, condensed milk, vanilla, cinnamon, ginger, nutmeg, and eggnog until combined. Begin adding sugar, slowly, and continue adding and whisking until all the sugar has been added and the buttercream reaches smooth peaks.
When the cookies are no longer warm, pipe or spread some of your filling onto the insides of half of the cookies. Top with another cookie to create a sandwich and enjoy!
Recipe by Jenna Gustin at Butter Loves Company