Preheat oven to 350°F. Prepare a standard cupcake pan with 12 paper liners. 

In a large bowl or stand mixer, beach together butter and sugar until light and fluffy. Mix in egg and vanilla until fully combined. Scrape bowl as needed. Add salt, baking soda and cocoa powder, followed by flour. Mix until smooth. With mixer running on low, add sour cream. Finally, slowly drizzle in warm water, mixing until no streaks remain. Use a large cookie scoop to evenly divide batter between 12 standard cupcake liners. 

Bake for 20-22 minutes or until a toothpick inserted comes out clean. Cool in pan for 3-5 minutes, then transfer to a wire rack to cool completely. 

When cupcakes have cooled completely, prepare frosting. In a large bowl or stand mixer, beat together butter and shortening until fully combined. Mix in vanilla and powdered sugar, followed by heavy cream. Mix until light and fluffy and fully combined. Transfer to a large piping bag, fit with desired piping tip. Pipe frosting onto cooled cupcakes. Immediately decorate with sprinkles. 

Store cupcakes in an airtight container for up to several days.