Preheat and Prep
Preheat oven to 375°F (190°C).
Line baking sheets with parchment paper or a silicone baking mat.
Cream Butter and Sugar
In a stand mixer or with a hand mixer, beat softened butter, granulated sugar, and brown sugar on medium speed 2–3 minutes until light, fluffy, and pale.
Scrape down the sides of the bowl.
Add Wet Ingredients
Add eggs and egg yolk one at a time, mixing well after each.
Add vanilla extract, almond extract (if using), lemon zest (if using), and sour cream (if using).
Mix on low until just combined.
Mix in Dry Ingredients
In a separate bowl, whisk together flour, cream of tartar, baking soda, baking powder, salt, and cornstarch (if using).
With mixer on low, slowly add dry ingredients to wet mixture; mix until a smooth dough forms. Do not overmix.
Fold in Sprinkles
Add jimmies or confetti sprinkles and fold in gently with a spatula until evenly distributed.
Stop as soon as sprinkles are mixed in to avoid color bleeding.
Scoop and Press
Use a cookie scoop or spoon to roll dough into 1½‑inch balls (about 1.5–2 tbsp each).
Place 2 inches apart on prepared baking sheet.
Dip the flat bottom of a glass in granulated sugar and press each ball down to about ¼‑inch thickness.
Optionally, roll the tops of dough balls in extra sprinkles before pressing for a more colorful finish.
Bake
Bake 10–12 minutes until edges are just set and slightly golden; centers should look slightly underdone.
Do not overbake — cookies will firm up as they cool.
Cool
Let cookies cool on the baking sheet 5 minutes before transferring to a wire rack.
Cookies reach their final soft, chewy texture once fully cooled.