Instructions

• Cream the unsalted butter and both sugars until light and fluffy, about 3-5 minutes. Then mix in the medium egg and vanilla extract until well combined.
• Sift together the all-purpose flour, baking soda, cream of tartar, and kosher salt. Blend this mixture into the wet ingredients until just combined to form a thick dough.
• Scoop tablespoon-sized dough balls and roll them in cinnamon sugar to coat. Place on a parchment-lined baking sheet and use your thumb or a teaspoon to create a thumbprint in the center.
• Preheat your oven to 350°F (175°C) and bake the cookies for about 7 minutes, or until they are just set. Gently press down on the thumbprint if needed.
• In a microwave-safe bowl or double boiler, melt the white chocolate with the eggnog until smooth. Stir in the optional rum and grated nutmeg if using, for extra festive flavor.
• Once the cookies have cooled completely, use a teaspoon to fill each thumbprint with the creamy ganache. Allow to set at room temperature or refrigerate until firm.