1. Preheat oven to 350°F (180°C). Line a standard cupcake pan with 12 baking cups and a second pan with 6 cups.
2. Sift together the flour, baking soda and salt on a sheet of wax paper.
3. Beat the butter on medium speed until fluffy. Add the sugar. Beat on medium speed until well mixed.
4. Add the eggs one at a time, mixing slowly after each addition.
5. Combine the vanilla extract and milk in a large liquid measuring cup.
6. Reduce the speed to low. Add one third of the flour mixture to the butter mixture; then gradually add a third of the milk mixture, beating until well incorporated. Add another third of the flour mixture, followed by a third of the milk mixture. Stop to scrape down the bowl as needed. Add the remaining flour mixture, followed by the remaining milk mixture, and beat until just combined.
7. Add the cocoa powder, beating on low until just incorporated.
8. Using a standard size ice cream scoop, fill each cupcake paper with batter, so that the wells are two-thirds full. Bake for 18–20 minutes (start checking at 15 minutes) or until a toothpick inserted into the center of a cupcake comes out clean. Transfer the pans to a wire rack to cool completely.