With your mixer, cream together sugar, butter, and shortening. Add in eggs and mix well.
In a separate large bowl, sift together flour, cream of tartar, baking soda, and salt.
Combine the wet ingredients into the dry and mix until all of the ingredients are incorporated together.
Stir in 3/4 cup white chocolate chips.
Place the dough in the freezer to chill for 30 minutes.
Pre-heat your oven to 350 degrees.
In a small bowl combine 1/4 cup sugar with 2 teaspoons ground cinnamon.
Measure 1/4 cup of cookie dough and use your hands to roll it into a ball. Roll the ball around in the cinnamon sugar mixture.
Place the balls of cookie dough on an ungreased baking sheet. Press additional white chocolate chips onto the outside of each cookie dough ball. Be sure to leave ample room between each cookie. I was able to get about 6 cookies on each baking sheet.
Bake for about 14-16 minutes. When just the edges of the cookies start to brown, you know they are done. The middle part of each cookie is going to appear undercooked. This is what you want!
Allow the cookies to cool on the baking sheet for about 10 minutes before transferring them to a wire rack to cool completely. As the cookies cool, they will lose the appearance that make them look like they are undercooked. Make sure to allow the cookies to cool completely!
Store leftovers in an air-tight container.