Ube Vanilla chiffon cake is made of of ube and vanilla chiffon slices, held together by a thin layer of ube buttercream.
Preheat oven to 350 F. Grease two 9x13 inch rectangular pan and line them with pre-measured parchment paper. Grease the parchment paper. In a large bowl, combine the flour, baking powder, salt, and sugar.
Pour the batters into the two prepared pans. Bake at 350 F for 12-15 minutes or until a toothpick inserted in the middle of the cake comes out clean. Please note that one cake may bake faster than the other so you need to watch closely. Transfer the cakes onto wire racks, including the parchment paper backing. Let cool completely.
After the cakes have cooled, we need to remove the parchment paper backing on them. To do this, sprinkle some powdered sugar on top of a cake. Top the cake with a piece of parchment paper that is about the same as its size. Top it with a baking pan that has the same size as well. Invert everything over, and carefully peel off the parchment paper on the cake. Reposition the parchment paper, top the cake with another baking pan, and invert again so that the cake is facing up as before. Repeat with the other cake. The cakes should be resting on inverted baking pans.
Spread the buttercream on top of the heavier cake. Carefully lift the other cake and place it on top of the buttercream. If desired, trim the edges of the cake. Cut the cake vertically in half, then cut into seven portions horizontally. You can either serve them on a platter if serving immediately, but I like to wrap them individually in plastic wrap for gifting or storing. Enjoy! These can be stored at room temperature, covered or wrapped for up to 2 days. For longer storage, keep them in the fridge for up to a week.
In a small bowl, whisk the softened butter until fluffy. Add in the sugar and whisk until incorporated. Add the milk and ube flavoring and whisk mixture together until smooth and spreadable. You can adjust the amount of milk to achieve desired consistency. You may add more sugar if mixture is too thin.