Preheat oven to 350°F (177°C). Line a baking sheet with a silicone baking mat or spray with nonstick cooking spray; set aside.
Whisk together flour (2 1/2 cups), baking soda (1/2 teaspoon), cream of tartar (1/2 teaspoon), and salt (1/8 teaspoon); set aside.
In another mixing bowl, beat together powdered sugar (1/2 cup), granulated sugar (1/2 cup), and butter (1/2 cup) with an electric mixer until light and fluffy, about 1 minute. Beat in oil (1/2 cup), eggs (1 large), vanilla (1/2 teaspoon), and orange zest (3 tablespoons). Gradually add the flour mixture and mix just until incorporated.
Roll the dough into ping-pong-sized balls. Flatten with the bottom of the glass dipped in sugar.
Bake for 7-8 minutes or until the center is set. Cookies will not brown! Don't overbake.
Make Frosting:
Meanwhile, make the frosting; with an electric mixer, beat butter (1/2 cup) until smooth and creamy. Add powdered sugar in 1/2 cup increments until combined. Add orange juice (1-2 tablespoons) and mix until smooth and spreading consistency. Spread over completely cooled cookies and top with sprinkles.
Notes
Cookie Scoop: For uniform size, I like to use a cookie scoop, then roll them in my hands to finish shaping them.
Room Temperature: For best results, have all your ingredients at room temperature. They mix together best this way.
Correct Flour Measuring: Use the spoon and sweep method to measure the flour. Here’s how:
Fluff the flour in the container with a whisk
Use a large spoon to spoon the flour from the container into the measuring cup, rather the dipping the cup into the flour and scooping it out.
Use a small offset spatula to sweep off the excess flour.