In a medium bowl, whisk together 2 1/2 cups cake flour, 2 1/4 teaspoons baking powder, 1 teaspoon vanilla bean powder, and 3/4 teaspoon kosher salt. With the mixer running on low, add the flour mixture to the butter mixture in three additions, alternating with adding 1 cup whole milk in two additions, starting and ending with the flour mixture. Mix on low speed until just incorporated — be careful not to overmix! It's okay to have one or two flour streaks left at this point.