2.

For the Dough: Adjust oven rack to middle position and preheat to 350°F (177°C). Using the same bowl and paddle attachment used for frosting (no need to clean), combine butter, sugar, baking powder, and salt. Mix at low speed to moisten, then increase speed to medium and beat until fluffy, pale ivory, and soft, about 5 minutes. Meanwhile, whisk egg whites, cream, and vanilla in a glass measuring cup until homogeneous. With the mixer still running, add liquid mixture in 4 or 5 additions, pausing to scrape bowl and beater about halfway through. Decrease mixer to low speed, sprinkle in cake flour, and continue mixing to form a soft dough. Fold batter once or twice with a flexible spatula to make sure it's well mixed from the bottom up.