Preheat the oven to 350 degrees Fahrenheit. Place the softened butter and sugars in a large bowl. Beat together at medium speed until light and fluffy, about 4 to 8 minutes. Reduce speed to low and beat in the vanilla at low speed. Beat in the eggs, one at a time In a separate bowl, sift together flour, cocoa powder, baking soda, and salt. Pour the flour mixture into the butter mixture gradually while beating until a smooth dough is formed. Mix in the chocolate chips using a wooden spatula. Drop the spoonfuls of cookie dough onto a cookie sheet, setting them 2 inches apart. Bake for 10 to 12 minutes. Let the cookies cool for 10 minutes on the cookie sheet before transferring to a serving plate. Enjoy with a glass of milk.