Preheat your oven to 350°F and line a baking sheet with parchment paper.
In a small/medium bowl, whisk together the ¾ cup all-purpose flour, ¾ teaspoon cinnamon, ½ teaspoon baking soda, and ½ scant teaspoon salt.
In a medium/large bowl, cream together the softened ¼ cup unsalted butter and ½ cup granulated sugar. Mix in the ¼ cup sourdough discard, 1 large egg yolk, and 1½ teaspoons vanilla extract really well.
Gently stir the dry ingredients into the wet ones until combined.
In a small bowl, combine 2 tablespoons granulated sugar with 1 teaspoon ground cinnamon.
Scoop dough balls & roll them in cinnamon sugar mixture. The dough will be sticky, but rolling them in the cinnamon sugar will make them easier to handle!I recommend using a 3-tablespoon cookie scoop.
Place the snickerdoodle dough balls about 2 inches apart on your prepared baking sheet. Bake on the middle rack until the edges are light tan and centers look a tiny bit gooey (about 9-11 minutes).
Let them cool on the baking sheet until they're firm enough to transfer to a wire cooling rack.
Video
Notes
Storage: Store cooled snickerdoodles in an airtight container for up to 5 days. Add a slice of bread to the container to keep them soft longer (this trick works like magic)! Or, store them in the freezer for up to 2 months.