I can’t think of a bigger crowd pleaser than a buttercream-frosted sprinkle cookie; everyone loves them! Sour cream adds flavor and lightness to these cookies, while a touch of white chocolate adds sweetness and body to the buttercream.
Preheat the oven to 350 degrees F. Line 2 sheet pans with parchment paper.
For the cookies: Combine the butter, granulated sugar, confectioners’ sugar, baking powder, salt and vanilla in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until a uniform, grainy paste forms, 2 to 3 minutes. Scrape the bowl clean, add the egg yolk and beat on medium speed until combined, about 30 seconds. Add the sour cream and beat until fluffy, about 30 seconds.
Whisk together the flour and cornstarch if using in a separate medium bowl. Add to the butter mixture all at once and mix on low speed until most of the flour is incorporated. Add the sprinkles and mix just until combined.
Scoop 10 rounded 1-tablespoon portions of dough onto each sheet pan and roll into smooth balls. Flatten the dough balls so they’re about 3/4-inch thick. Bake one sheet at a time until the cookies are light golden on the edges and set in the center, 15 to 17 minutes. Place the pans on wire racks and allow the cookies to cool completely on the pan.
For the frosting: Place the chopped white chocolate in a small microwave-safe bowl and microwave in 12-second increments, stirring after each, until completely melted, about 36 seconds. Set aside.
Beat the butter in the bowl of a stand mixer with the paddle attachment until smooth and shiny, 2 to 3 minutes. Scrape the bowl and paddle clean then add the confectioners’ sugar, vanilla, salt and water. Mix on low to combine, then raise to high speed and beat 1 minute. Scrape the bowl and paddle clean and add the warm melted chocolate to the bowl, mixing on low until combined. Raise the mixing speed to medium and beat until smooth, about 1 minute.
Place the buttercream in a pastry bag fitted with a star tip and pipe a star or rosette onto the center of each cookie. Alternately, use a small spatula to spread a heaping tablespoon of buttercream onto each. Finish the cookies by sprinkling rainbow sprinkles over the buttercream. Store the cookies in an airtight container for up to 3 days.
Cornstarch is optional, but it makes for a fluffier cookie. You can chill these cookies before storing them to firm the chocolate in the buttercream and make them neater to stack.