1 1/2 cups unbleached all-purpose flour (7 1/2 ounces)
1 cup granulated sugar (7 ounces)
1 1/2 teaspoons baking powder
1/2 teaspoon table salt
8 tablespoons unsalted butter (1 stick), room temperature
1/2 cup sour cream
1 large egg , room temperature
2 large egg yolks , room temperature
1 1/2 teaspoons vanilla extract
Chocolate Ganache Frosting
1 cup heavy cream
8 ounces chocolate (milk, semisweet or bittersweet) , chopped
For Cupcakes: Adjust oven rack to middle position; heat oven to 350 degrees. Line standard muffin/cupcake tin with paper or foil liners.
Whisk together flour, sugar, baking powder, and salt in bowl of standing mixer fitted with paddle attachment. Add butter, sour cream, egg and egg yolks, and vanilla; beat at medium speed until smooth and satiny, about 30 seconds. Scrape down sides of bowl with rubber spatula and mix by hand until smooth and no flour pockets remain.
Divide batter evenly among cups of prepared tin using 2-ounce ice cream scoop (I used #20 size) or heaping tablespoon. Bake until cupcake tops are pale gold and toothpick or skewer inserted into center comes out clean, 20 to 24 minutes. Use skewer or paring knife to lift cupcakes from tin and transfer to wire rack; cool cupcakes to room temperature, about 45 minutes.
For Frosting: Bring cream to boil in small saucepan or in microwave. Place chocolate in medium bowl and pour boiling cream over. Cover with foil and let stand 5 minutes. Whisk until smooth, then cover with plastic wrap and refrigerate until cool and slightly firm, 45 minutes to 1 hour.
Transfer cooled chocolate mixture to bowl of standing mixer fitted with whisk attachment; whip mixture at medium speed until fluffy and mousse-like and forms medium-stiff peaks, about 2 minutes.
Pipe the frosting on to the cooled cupcakes with a large star tip. Or spread 2 to 3 generous tablespoons with a knife. Decorate with sprinkles, jimmies, etc.