Pre-heated oven to 190°C.
Sift the flour, baking powder, sugar and cocoa into a bowl.
Add the chocolate and walnuts then form a well in the centre. Pour the butter, milk, egg and vanilla into this and stir until just combined. Spoon the mixture into greased muffin tins until 3/4 of the way up the well.
Place in an oven and bake for 20 minutes, or until done.