Preparation Steps

• In a medium bowl, whisk together the flour, baking soda, cream of tartar, and salt. Set aside.
• In a large bowl, whisk the melted butter, granulated sugar, and brown sugar until smooth.
• Add the egg, egg yolk, and vanilla extract to the butter mixture, whisking until well combined.
• Gradually fold the dry ingredients into the wet ingredients using a rubber spatula until just combined. Do not overmix.
• Cover the bowl with plastic wrap and chill the dough in the refrigerator for at least 2 hours.
• Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
• Scoop about 1.5 tablespoons of dough per cookie, roll into balls, and coat generously in the coarse sugar.
• Place the dough balls on the prepared baking sheets, spacing them about 2 inches apart.
• Bake for 9-11 minutes, or until the edges are just set but the centers still look slightly underbaked.
• Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.