Adjust oven rack to middle position and preheat oven to 350°F. Line two 12-count cupcake tins with paper liners. Set aside.
In a medium bowl, sift together flour, baking soda, and salt.
2¼ cups cake flour, 2 teaspoons baking soda, 1 teaspoon kosher salt
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and brown sugar together until incorporated, about 2 minutes.
½ cup unsalted butter, 2½ cups brown sugar
Add the eggs, one at a time, beating to incorporate after each addition.
3 large eggs
With the mixer on low speed, add the vanilla and melted chocolate.
1½ teaspoons pure vanilla extract, 1 ounce unsweetened chocolate
Add the sour cream and dry ingredients alternately in 3 additions, starting and ending with the flour. Beat each addition just until incorporated.
1 cup sour cream
Stir in the hot water and beat just until will mixed.
1 cup water
Pour the batter (it will be very runny) into the prepared muffin tins, filling each well about ¾ full.
Place one cupcake pan in the oven and bake for 10 minutes. Rotate the pan and bake another 5-10 minutes, or until a toothpick inserted in the middle comes out clean.
Transfer the cupcake pan to a cooling rack and place the second cupcake pan into the oven. Bake as directed above.
Cool the cupcakes in the baking pan for 5 minutes. Carefully remove the cupcakes and place on the cooling rack to completely cool.
Frost each cupcake with chocolate fudge frosting and garnish with your favorite sprinkles.
4 cups Chocolate Fudge Frosting, Sprinkles