For the Cupcakes:

Cup of Yum
1. Preheat the oven to 350 degrees (F). Line two-12 mold cupcake pans with paper liners. Spray the liners lightly with nonstick baking spray and set aside. 
2. In a large mixing bowl, combine the cake flour, baking powder, and salt. Set aside.
3. In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld electric mixer, beat the butter on medium speed until creamy, about 1 minute. 
4. Gradually add in the sugar and beat until light and fluffy, about 3 minutes, scraping down the sides and bottom of the bowl as needed. 
5. Beat in the eggs and egg yolks, one at a time, beating well after each addition, and scraping down the sides and bottom of the bowl as needed. Beat in the vanilla. Turn the mixer off for a moment. 
6. In a glass measuring cup, combine the buttermilk and sour cream. Whisk well to combine. Set aside. 
7. With the mixer running on low, add the flour mixture in three additions, alternating with the buttermilk mixture in two additions, beginning and ending with the flour, and mixing just until the flour is incorporated.
8. Turn the mixer off. Use a rubber spatula to scrape the bottom and sides of the bowl once or twice, ensuring all ingredients are evenly combined. 
9. Divide the batter evenly among the prepared pans, filling each mold halfway full. You will have enough batter leftover to bake an extra two to three cupcakes, if desired. Or you may discard any excess batter. 
10. Bake for 18 to 20 minutes, or until the tops are golden brown and a toothpick inserted in the center of a cake comes out clean, or with moist crumbs attached. 
11. Cool the cupcakes in the baking pans, placed on cooling racks, for 5 minutes. Then remove the cupcakes and transfer them to a cooling rack to cool completely. 

For the Frosting:

1. Place both chocolates in a microwave-safe bowl. Heat in the microwave, on low power, in 30 second increments, stirring between each increment, until the chocolate is completely smooth and melted. You may also do this in a double-boiler. Set aside and cool until tepid. 
2. In the bowl of a stand mixer fitted with the paddle attachment or in a large bowl using a handheld electric mixer, beat the butter until completely smooth.
3. On low speed, gradually add in the confectioners' sugar, beating until each addition has been completely incorporated into the butter before adding more. Once all of the sugar has been added, increase speed to medium-high and beat for 3 minutes. 
4. Decrease the speed back to low and beat in the vanilla extract. Then slowly pour in the cooled melted chocolate and continue mixing until well combined, tuning the mixer off and scraping down the sides and bottom of the bowl as needed. Once all of the chocolate has been added, increase the speed back to medium-high and beat for 2 minutes. Set aside for 30 minutes to firm up, or up to 3 hours. Do not refrigerate (the chocolate will seize and render the frosting impossible to spread). 
5. Once all of the chocolate has been added, increase the speed back to medium-high and beat for 2 minutes.
6. Set aside for 30 minutes to firm up, or up to 3 hours. Do not refrigerate (the chocolate will seize and render the frosting impossible to spread). 

Assembly:

1. Scrape your chocolate frosting into a piping bag fitted with an open star piping tip. 
2. Pipe the frosting on top of each cupcake. Sprinkle with nonpareils and serve!