Preheat oven to 350°F/175°C. Put 12 cupcake cases in a 12-cupcake tray.
In the bowl of an electric mixer with the paddle attachment, cream the butter until smooth. Add the sugars and beat until fluffy and light, about 3 minutes. Add the eggs, beating well after each addition. Pour in the chocolate.
In a small bowl, whisk together the flour and baking soda. In a jug, mix the milk and vanilla together.
Add the dry ingredients and buttermilk/vanilla in three additions, beginning and ending with dry ingredients.
Fill the cupcake liners ¾ of the way, and bake for 20-25 minutes, a toothpick inserted should come out clean