Preheat the oven to 350°F / 180°C / Gas Mark 4. Line cupcake pans with papers.
In a medium bowl, whisk cocoa and hot water together until smooth.
Let cool slightly, then whisk sour cream and melted chocolate into cocoa mixture until combined.
In a separate bowl, sift together flour, baking soda, baking powder and salt; set aside.
In a separate mixing bowl, using an electric mixer, cream butter and sugar until light and fluffy, 3 to 4 minutes. Add eggs, one at a time, beating to combine after each. Scrape down sides of bowl. Beat in vanilla.
With mixer on low speed, add flour mixture in two parts, alternating with the cocoa mixture (beginning and ending with the flour). Beat until combined, and you no longer see pockets of flour. Use a rubber spatula to fold the batter a few times, making sure to scrape the bottom of the bowl to combine thoroughly. Fill the cupcake liners ⅔ full (I use a scoop for this to make sure they are consistent). Bake for 18 - 20 minutes, or until a toothpick inserted comes out with only a few moist crumbs. (timing will depend on your oven)
Let cool completely before frosting.