In a large bowl, blend butter, shortening, and sugar with hand mixer. Add egg, egg yolk, vinegar, vanilla, and food coloring and blend. Food coloring may stain.
In a medium bowl, combine dry ingredients: flour, cocoa powder, cornstarch, baking soda, and salt.
Add dry mixture to butter mixture and stir to combine.
Fold in approx. ⅔ of the white chocolate chips.
Scoop dough out by heaping tablespoons and place on cookie sheets, evenly spaced. Press tops of cookies down slightly with a finger.
Bake 8-9 minutes at 375°. Immediately after removing from the oven, press remaining white chocolate chips into the top as desired.