• Preheat oven to 350 F. Line a muffin pan with cupcake liners, and line a second one with 5 liners. This recipe makes 17. Cream butter, baking powder, salt, sugar and ube flavor in the bowl of a stand mixer for about 5 minutes until the ingredients are fluffy and incorporated.
• With the mixer on low speed, add the egg whites one at a time and mix for a few seconds until incorporated fully into the batter.
• Add ⅓ of the flour and continue mixing. Add in half of the milk, mix then add another third of the flour. Add the remaining half of milk, mix until incorporated and finally add the remaining third of the flour. Mix batter for a few more minutes just until smooth. Spoon into the cups until they are 2/3 full. Bake in a 350 F oven for 15-18 minutes or until a toothpick inserted at the center of a cupcake come out clean. Cool cakes completely in a wire rack before frosting.

For the Swiss Meringue buttercream:

• Fill a cooking pot with about 2-3  inches of water. Bring the water to a boil, reduce the heat and let it simmer. Place a heat-proof glass bowl on top of the pot of simmering water. It should be a bowl that fits perfectly over the pot. The simmering water should not be touching the bottom of the bowl. If it does, you need to lessen the amount of water carefully. 
• Add the egg whites and sugar to the bowl and start whisking continuously using a wire whisk. Once the mixture turns frothy and white and reaches a temperature of 160 F, turn off the heat. That should take about ten minutes or so. Pour the egg whites mixture into the bowl of a stand mixer fitted with the whisk. Start whisking on high speed until the mixture forms shiny and firm peaks. This should take around 5-8 minutes. The bowl should be cold to the touch.
•  Add in the softened butter to the mixer and continue whisking. The buttercream might turn lumpy or might curdle for a moment. Keep whisking until a thick, fluffy icing takes form. Add in the ube flavoring and continue whisking until combined. Store in a tightly closed container at room temperature or use icing right away.