Lightly grease a mini muffin tin with a little butter or a quick spray of cooking oil and set aside. In a large mixing bowl, add the butter and sugars and beat with an electric mixer until combined. Add the vanilla and the egg and beat until mixture is creamy and smooth. Sift in the flour, cocoa powder and baking soda and stir using a wooden spoon or spatula until the mixture comes together to form a dough. Cover with plastic wrap and chill the cookie dough for 1 hour.
Preheat the oven to 180 degrees (360 F). Using your hands, roll the dough into small balls and place each one into the mini muffin tin. Continue until you run out of dough and have filled approximately 20 mini muffin holes. Gently press down so the dough fills the cavity and has a flat top. Then grab a rolling pin dusted with cocoa and press down and twist on top of the dough to create a hole or cavity in the middle of each cookie.
Place the tray in the oven for approximately 10 minutes. Remove from the oven. At this point, the cookies will have puffed up quite a bit, so grab your rolling pin again and very gently reinforce the hole in the middle of each cookie. Set aside to cool for 10-15 minutes and then gently remove the cookies from the tray and place them onto a cooling wire rack. It's ok if the cookies appear really soft, especially in the middle, as they will firm up when they cool.
To make the cookie dough - roughly chop the butter into chunks and place in a large mixing bowl, along with the sugar. Using an electric beater, cream the butter and sugar until combined. Add the vanilla and beat for 30 seconds or so, until smooth and creamy. Then add the flour and stir with a wooden spoon. Add the milk, one tablespoon at a time, until the dough reaches your desired consistency (I just used three tablespoons of milk). Stir in the chocolate chips. Cover and place the cookie dough in the fridge for at least 15 minutes, then once the cookies have cooled, spoon a teaspoon of cookie dough into each one.