1. Prep: Preheat oven to 375°F (190°C). Line baking sheets with parchment paper.
2. Mix dry ingredients: Whisk flour, baking soda, and salt in a bowl.
3. Cream wet ingredients: Beat butter and sugar until light and fluffy (~3 min). Add egg, sour cream, and vanilla; mix until smooth.
4. Combine: Gradually add flour mixture until just incorporated. Fold in sprinkles.
5. Chill dough 30–60 minutes (essential for thick, soft cookies).
6. Scoop: Roll into 3-tbsp balls (or use a #20 cookie scoop). Place 3″ apart on sheets.
7. Bake 9–11 minutes, until edges are set but centers look soft and slightly underdone.
8. Cool on pan 5 minutes before transferring to a rack.

💡 Pro Tips:
– Use jimmies, not nonpareils—they hold color and don’t bleed.
– Don’t overbake—they’ll firm up as they cool.
– For extra softness, store cooled cookies with a slice of bread in an airtight container.

Prep Time & Nutrition (per cookie, makes 16):