Start by adding the flour, powdered sugar, cornstarch, brown sugar, and granulated sugar to a large mixing bowl. Stir everything together until well combined. This step ensures the sugars and starch are evenly distributed throughout the dough.
Pour the melted butter, vanilla extract, and milk into the bowl. Mix until a soft dough forms. At first, the mixture may look crumbly, but it will come together as you continue stirring.
Gently fold in the mini chocolate chips. Make sure they are evenly spread throughout the dough so every bite has plenty of chocolate. Avoid over-mixing to keep the dough tender.
Use a cookie scoop or tablespoon to portion the dough. Roll gently into bite-sized balls and place them on a tray or plate. Chill in the refrigerator for about 10 minutes so they firm up.
Melt the dark chocolate until smooth. Dip each chilled bite halfway into the chocolate or drizzle it over the top. Let the chocolate set before serving for the best texture.