01 -
Preheat oven to 180°C (350°F) and line several baking sheets with parchment paper.
02 -
In the bowl of a stand mixer fitted with the paddle attachment, combine butter, oil, granulated sugar and powdered sugar. Sprinkle baking soda, cream of tartar and salt evenly across the mixture. Beat until well combined and creamy, about 1-2 minutes, scraping down the sides as needed.
03 -
Incorporate sour cream, eggs and vanilla, mixing thoroughly for 1-2 minutes until well combined.
04 -
Add flour gradually and mix until evenly distributed throughout the dough.
05 -
Scoop dough into large rounded tablespoons and roll into balls. Place several centimeters apart on prepared baking sheets.
06 -
Place sugar in a small bowl. Spray the bottom of a flat-bottomed glass with cooking spray, dip into sugar, then press each cookie to about 0.6-1.3cm thickness. Continue with remaining cookies (no need to respray the glass between cookies).
07 -
Bake for 10-12 minutes or until just set. For soft, creamy cookies, avoid over-baking; they should not become golden on edges or bottom.
08 -
Allow cookies to cool on baking sheets for a few minutes before transferring to a cooling rack to cool completely.
09 -
In a clean mixer bowl, combine butter, sour cream, and vanilla. Beat until smooth and thick, about 1-2 minutes. Add powdered sugar and 1 tablespoon of milk or cream, mixing until creamy. Add additional milk in small increments as needed to achieve desired consistency.
10 -
Spread frosting on completely cooled cookies and add sprinkles as desired.