Preheat your oven to 350F. Line the bottom of 3 8-inch round baking pans with parchment paper and set aside.
In a large bowl, sift 2 1/2 cups cake flour, 2 1/2 teaspoons baking powder and 1/2 teaspoon salt. In a smaller bowl, combine 2 teaspoons ube extract, 1/2 cup vegetable oil and 1/2 cup milk. Set aside.
In another large bowl, using a handheld electric mixer, beat egg yolks while gradually adding 1/3 cup sugar until the mixture is light yellow (about 7 minutes on medium-high speed). Add 1/2 cup ube jam and beat until smooth and no big lumps remain. If you want your cake to be a vibrant purple, you can add gel food colour at this stage and give the batter a stir until the colour is evenly distributed.
Fold a third of your flour mixture into your egg mixture just until combined. Then add half of your milk mixture, another third of your flour, the rest of your milk and finally the rest of your flour, folding each addition just until combined. Be careful not to overwork the batter.
Using a stand mixer with the whisk attachment (or your handheld electric mixer - making sure that the beaters are clean, dry and grease-free), beat the 8 egg whites on medium-high speed until they start to get foamy. Then add 1 teaspoon cream of tartar and beat until very frothy.
Gradually add 3/4 cup sugar into egg whites until you reach stiff peaks. Then take about 1 1/2 cups of the meringue and fold it into your cake batter to lighten it. Then fold your cake batter into the meringue in thirds until incorporated.
Transfer the batter to prepared pans and bake for 20-30 mins or until a toothpick inserted in the centre comes out with minimal crumbs.
Put the pans upside down on a cooling rack and cool completely.
Once your buttercream has come together, start adding 1/4 teaspoon ube extract at a time until you get the flavour you desire (I put 1 teaspoon total). Whip until the flavour is fully incorporated and the buttercream is evenly coloured.
Trim the tops of your cakes if they are uneven. Trim the sides too to get rid of the brown edges so you're left with a vibrant purple cake.
Fill and decorate with frosting as desired.
You can find ube extract in Asian supermarkets. And there's always Amazon (see below).
You may need to adjust the sugar content depending on the kind of ube jam you find. Ube jam can be found in Asian supermarkets as well.