To make the cupcakes, preheat oven to 350 and line 2 cupcake pans with paper liners.
In a small bowl or measuring cup, whisk together cocoa powder and hot water until smooth.
In a medium bowl, whisk together flour, baking soda, baking powder, and salt, and set aside.
Within a medium saucepan over medium heat, stir together butter and sugar. Heat, stirring occasionally, until the butter is melted and the mixture is smooth.
Transfer to the bowl of a stand mixer or a large mixing bowl, and beat until the mixture is cool, 4-5 minutes.
Add eggs one at a time, mixing well after each addition.
Add vanilla and cocoa mixture, and blend until smooth.
With the mixture on low speed, add the flour mixture in three additions, alternating with the sour cream. Mix until smooth.
Scoop into prepared pans, filling 2/3 full.
Bake until a toothpick comes out clean, about 15-18 minutes. Let cool.