Preheat the oven to 170°C. Line 24 muffin tins with cupcake papers.
Sift the flour, cocoa powder, baking soda and salt into a bowl and set aside
Cream the butter and sugar until light and fluffy. Add the eggs one at a time until thoroughly mixed through.
In a separate bowl mix the sour cream, milk and vanilla extract.
With the mixer on low speed, alternately add the dry ingredients and the sour cream mixture, beginning and ending with the dry ingredients. Mix until just incorporated.
Spoon the mixture into the prepared muffin tins and fill approximately 3/4 full
Bake in the oven for 18-20 minutes or until the cupcakes slightly spring back when touched.
Allow the cupcakes to cool slightly in their tin before turning out onto a wire rack.