Preheat your oven to 350 degrees. Line a cupcake pan with cupcake liners.

In a small saucepan, combine the butter, cocoa powder, salt, and water and put over medium heat. Cook, stirring, just until melted and combined. Remove from the heat and set aside.

Put the flour, sugar, and baking soda in a large bowl and whisk to combine. Add half of the melted-butter mixture and whisk until completely blended (it will be very thick). Add the remaining butter mixture and whisk. Add the eggs one at a time, stirring each to blend completely before adding the next. Add in the sour cream and vanilla until smooth.

Scoop batter into prepared cupcake liners. Fill each liner 3/4 of the way. Place in oven and bake for 30 minutes or until a toothpick inserted in the middle comes out clean. Let cool

In a stand mixer bowl with paddle attachment, beat the butter at medium-high speed until smooth, about 20 seconds. Using a paring knife, scrape seeds from vanilla bean into butter and beat mixture at medium-high speed to combine, about 15 seconds.

Add powdered sugar and salt; beat at medium-low speed for about 1 minute. Scrape down the bowl and beat at medium speed until mixture is fully incorporated, about 30 seconds; scrape bowl, add vanilla and heavy cream, and beat at medium speed until incorporated, about 20 seconds, then increase speed to medium-high and beat until light and fluffy, about 4 minutes, scraping down bowl once or twice. If you would like your buttercream thicker, add more powdered sugar, and if you want it thinner, add more milk.

Once cupcakes have cooled, top with desired amount of buttercream and sprinkles!