Preheat the oven to 350°F / 180°C and line two baking sheets with parchment paper or a silicone baking mat.

In a large mixing bowl, combine 3 cups all-purpose flour, 1 teaspoon baking soda, 2 teaspoons cream of tartar, ½ teaspoon salt, ¾ teaspoon ground cinnamon, and ½ teaspoon ground nutmeg. Whisk until well combined and set aside.

In the bowl of a stand mixer with the paddle attachment, or in a large mixing bowl with a hand mixer, cream 1 cup softened unsalted butter with 1 cup white sugar and ⅓ cup dark brown sugar on medium speed until light and fluffy, about 2-3 minutes. Add the egg, yolk, ½ teaspoon vanilla extract, and 1 ½ teaspoons rum extract and mix on low speed until thoroughly combined.

Now add the dry ingredients into the wet ingredients, about ⅓ of the mix at a time, and mix on low speed until just combined. In a small bowl, combine the remaining ¼ cup white sugar, ¼ teaspoon ground cinnamon, and ¾ teaspoon ground nutmeg and whisk together.

Using a medium cookie scoop, scoop about 1 ½ tablespoons of the snickerdoodle dough and place onto the prepared baking sheet about 2 inches apart. Once the baking sheet is full, take one portion of the dough at a time and roll it in your hands to form a ball. Roll the ball through the cinnamon and nutmeg sugar and place back on the baking sheet.

Place the cookie sheet into the oven and bake for 9-11 minutes. The cookies should just barely be browning around the edges and almost look underbaked. Remove from the oven and use the bottom of a measuring cup or small bowl to gently press down the tops of the cookies slightly.

Allow them to cool on the sheet for about 5 minutes and then move to a cooling rack to finish cooling.

If you have two baking sheets you can place the next one in the oven as soon as you remove the first one. The baking sheet should be completely cooled before placing more cookies on it or the butter will melt prematurely.