Preheat the oven to 350°F (175°C). Prepare baking sheets lined with parchment paper.

In a stand mixer, combine softened butter, butter-flavored shortening, granulated sugar, and powdered sugar. Beat until light and fluffy, about 3 minutes.

Add room-temperature eggs to the creamed mixture and mix until well combined. Scrape down the bowl as needed.

In a separate bowl, whisk together cream of tartar, baking soda, salt, and flour.

Gradually mix dry ingredients into wet ingredients until a cohesive dough forms.

Using a cookie scoop, portion dough onto prepared baking sheets, rolling into balls and pressing down slightly.

Bake for 8-10 minutes, looking for golden edges with soft centers.

Let cookies cool completely on a wire rack.

To prepare the frosting, blend softened butter, sour cream, and a pinch of salt until smooth, then gradually add powdered sugar.

Frost cooled cookies generously using an offset spatula.