For the Yellow Cupcakes
1/2 cup + 2 tbsp buttermilk
1 large egg
2 large egg yolks
1 1/2 tsp vanilla extract
1 1/4 cups (163g) all-purpose flour
3/4 cups (155g) sugar
3/4 tsp baking powder
1/4 tsp baking soda
1/4 tsp salt
1/2 cup(112g) unsalted butter, room temperature
For the Chocolate Buttercream Frosting
1 1/4 cups unsalted butter, room temperature
3 3/4 cups + 2 tbsp (445g) powdered sugar
6 tbsp (44g) natural unsweetened cocoa powder
1 tsp vanilla extract
4–5 tbsp water or milk
pinch of salt
Preheat oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.
Combine the buttermilk, egg, egg yolks and vanilla extract in a large bowl, then separate about 1/4 cup of the mixture into another bowl or measuring cup. About 3/4 cup should remain in the other bowl. Set both aside.
In a large mixer bowl, combine the flour, sugar, baking powder, baking soda and salt.
With the mixer on the lowest speed, add the butter about a tablespoon at a time, allowing it to incorporate before adding the next tablespoon. As you add more butter, the mixture will start to clump together a bit and should end up resembling wet sand.
Add the larger of the reserved egg mixture (about 3/4 cup) to the dry ingredients/butter mixture. Stir on the lowest speed until it’s incorporated, then scrape down the sides of the bowl.
Increase the speed to medium high and beat until light and fluffy, about 45 seconds to a minute. Scrape down the sides of the bowl.
Turn the speed down to low and slowly add the remaining egg mixture in a slow stream until incorporated.
Scrape the sides of the bowl, then turn the speed back up and mix until well combined, about 10-15 seconds.
Fill the cupcake liners about 3/4 full and bake for 15-18 minutes, or until a toothpick inserted comes out clean.
Remove cupcakes from the oven and allow to cool for 2-3 minutes, then turn out onto a cooling rack to cool completely.
When cupcakes are cool, make the frosting. Add the butter to a large mixer bowl and beat until smooth.
Carefully pour in about half of the powdered sugar and mix until smooth and well combined.
Add the cocoa powder, vanilla extract and 2-3 tablespoons of water or milk and the salt to the frosting and mix until well combined.
Add the remaining powdered sugar and mix until smooth. Add additional milk or water as needed to get the right consistency.
Pipe the frosting onto the cupcakes.