Preheat oven to 350 degrees and lightly grease a baking sheet.
Mix together butter, cream cheese, and sugars until light and fluffy. Add vanilla, egg, and milk and mix well.
In a second bowl whisk together flour, cocoa powder, corn starch, salt, and baking soda.
Add dry ingredients to wet ingredients and mix until incorporated. If mixture is too dry and doesn't come together, you can add a couple more teaspoons of milk if needed. Stir in chocolate chips.
Form dough into 1 1/2 inch balls and place on baking sheet. (see note) Bake for 10-12 minutes (don't over bake) and allow to cool for at least 10 minutes on the baking sheet before transferring to a cooling rack to cool completely. Store in airtight container at room temperature.
I always like to press a few extra chocolate chips into the top of my cookie dough balls right before putting them into the oven - it makes them so pretty!