Ingredients
Equipment
Method
- Position oven rack in the middle and heat to 350°F (175°C), then line two light-colored aluminum baking sheets with parchment paper and set aside.
- In a medium bowl, whisk together 2¾ cups (330g) all-purpose flour, 1 tsp (4g) baking powder, and ½ tsp (3g) fine sea salt for 20 full rotations until evenly combined, then set aside.
- In the bowl of a stand mixer fitted with the paddle attachment, beat ½ cup (113g) softened unsalted butter and 1 cup (200g) granulated sugar on medium speed for exactly 3 minutes until pale ivory and nearly doubled in volume.
- With the mixer on low, add 1 large egg and mix for 20 seconds, then add 1 egg yolk and mix for another 20 seconds, then scrape down the bowl.
- Add ½ cup (120g) full-fat sour cream and 2 tsp (10ml) pure vanilla extract and mix on low for 20 seconds until just incorporated, then scrape down the bowl again.
- With the mixer on its lowest speed, add the flour mixture in three additions, allowing each to mostly disappear before adding the next, then stop the mixer the moment no dry streaks remain and finish with 10 folds of a rubber spatula from the bottom of the bowl upward.
- Using a 1.5-tablespoon cookie scoop, portion the dough into 45g balls, roll briefly between your palms, and space at least 2 inches (5cm) apart on the prepared pans — do not flatten.
- Bake one sheet at a time on the middle rack at 350°F (175°C) for 10 to 12 minutes, pulling the pan the moment the edges look set and lightly golden and the centers still look slightly puffed and soft.
- Leave cookies on the hot pan for exactly 5 minutes, then transfer to a wire rack and cool completely for a minimum of 30 minutes before frosting.
- In the clean bowl of the stand mixer, beat ½ cup (113g) softened unsalted butter on medium speed for 2 minutes until pale, then add 3 cups (360g) sifted powdered sugar in three additions on the lowest speed, followed by 2 tbsp (30ml) milk or cream, 1½ tsp (7.5ml) vanilla extract, and a pinch of fine sea salt, then increase to medium-high and beat for 2 full minutes until bright white and holding a stiff peak.
- Fit a piping bag with a Wilton 1M open star tip, fill two-thirds full with buttercream, and pipe a rosette swirl onto each cooled cookie starting from the center and spiraling outward, then immediately scatter rainbow sprinkles over each cookie while the frosting is still soft.
Notes
- Butter: must be softened to 65–68°F — too warm = spread flat, too cold = lumpy dough and dense cookies.
- Sour cream: use full-fat only at room temperature — low-fat has higher water content and causes more spread.
- Baking powder: test freshness by dropping ½ tsp into hot water — no bubbles means replace it before baking.
- Powdered sugar: sift before adding to the mixer — unsifted sugar creates grit that beating alone cannot eliminate.
- Flour: spoon into measuring cup and level — scooping packs up to 30g extra per cup and dries the dough.
- Storage: airtight hard-sided container at room temperature up to 3 days, refrigerator up to 6 days.
- Freeze frosted: flash-freeze on a baking sheet 1 hour, then transfer to freezer-safe container with parchment between layers, up to 2 months.
- Freeze unbaked dough: freeze portioned balls solid, store in freezer bag up to 3 months, bake from frozen adding 2–3 minutes.
- Make ahead: dough refrigerates up to 24 hours; buttercream refrigerates up to 3 days — re-beat 60 seconds before piping.
- Scaling: this recipe doubles reliably — baking powder does not scale linearly beyond a double batch; do not triple the leavening.
- Altitude: above 3,500 feet reduce baking powder to ¾ tsp, add 2 tbsp flour, bake at 375°F (190°C).
- Allergens: contains gluten, dairy, and eggs. For dairy-free, substitute plant-based butter (same weight) and full-fat coconut cream for sour cream — texture will be slightly less domed.
- Tip: pipe the frosting within 1 hour of the cookies cooling — the surface grips the buttercream better than a fully cold cookie.