Preheat your oven to 350°F and lightly grease a cookie sheet. This step is essential to ensure your cookies don’t stick.
In the stand of a mixer, cream together butter, vegetable oil, both sugars, sour cream, and eggs. Mix until well combined and fluffy. You should see the mixture turn lighter in color.
In another bowl, combine flour, baking soda, cream of tartar, and salt. Mixing these dry ingredients separately ensures even distribution.
Slowly add the dry mixture to your creamed mixture. Mix until combined. Be careful not to over-mix; you want your cookies to stay soft.
Roll the dough into 1½ inch balls and place them on the greased cookie sheet. This is where the fun begins!
Place ¼ cup sugar in a separate bowl. Lightly grease the bottom of a glass with cooking spray. Dip the bottom of the glass into the sugar and then press down onto each dough ball to make it about 3 inches in diameter. This gives the cookies their signature cracked edges.
Bake for 8 to 9 minutes. Look for a light golden color on the edges. Let the cookies sit on the cookie sheet for 5 minutes before removing them to a cooling rack.
While the cookies are cooling, cream together butter, powdered sugar, a drop of food coloring, and vanilla extract. Mix until smooth.
Add milk as needed to achieve the desired frosting thickness. I usually add 3 to 4 tablespoons until it’s just right.
Spread the frosting on the cookies right before serving. Enjoy the delightful combination of textures and flavors!