2 cups (4 sticks) unsalted butter, room temperature
2 teaspoons vanilla extract
1 vanilla bean, split lengthwise and scraped
To make the cupcakes - preheat the oven to 350 degrees. Line cupcake pans with liners and set aside.
In a medium bowl, sift together the cake flour, baking powder and salt. In another medium bowl, whisk together the buttermilk and egg whites. In the bowl of a stand mixer fitted with the paddle attachment, add the granulated sugar, vanilla bean seeds and butter. Beat on medium speed for 3 minutes, until it is very light. Add in the vanilla extract and beat until incorporated. With the mixer on low speed, add in one third of the flour mixture. Next, add in half of the buttermilk mixture, beating until incorporated. Repeat this process, one third flour mixture, remaining half buttermilk mixture. Finally add in the remaining third of the flour mixture. Once incorporated, increase the speed to medium low and beat for 2 minutes, to ensure it is well aerated. Fill the prepared cupcake tins evenly with the batter (about 2/3 of the way full).
Bake for about 18 minutes, or until a toothpick inserted into the middle comes out clean. Let the cupcakes cool for about 5 minutes in the pan, then remove to a wire rack to cool completely before frosting.
To make the frosting - add the egg whites, sugar and salt to a heatproof bowl. Set the bowl over a saucepan of simmering water. Whisk the mixture almost constantly until the temperature reaches 160 degrees (if you don't have a thermometer, the sugar should be completely dissolved and the mixture should feel completely smooth if you rub it between your fingers).
Carefully transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment. Start on low and gradually increase the speed to medium high. Whip until it is stiff and glossy, and the bottom of the bowl feels neutral to the touch (it should not be warm), about 10 minutes.
With the mixer on medium low speed, slowly add in the butter, a few tablespoons at a time. Mix well after each addition before you add more. Next, add in the vanilla and vanilla bean seeds and mix to combine. Switch to the paddle attachment and beat on medium until the frosting is smooth and has a silky texture (this usually takes a few minutes). If the mixture looks curdled, just keep beating and it will come back to a smooth texture.