Preheat oven to 350° F (175° C). Line the bottom of (2) 8-inch round pans (preferably nonstick aluminum 3-inch deep) with parchment paper. No need to grease the sides so the cake batter can “crawl” up the sides and rise properly.
Dry ingredients: In a bowl, combine flour, baking powder, and salt. Whisk and set aside.
Wet ingredients: In a large bowl, combine ube and ¼ cup water. Use a fork to mash the mixture until smooth. Then, add 6 egg yolks, ½ cup granulated sugar, oil, and ube extract. Whisk until well-combined, then set aside.
Sift half of the dry ingredients into the wet, folding gently using a whisk or spatula. Repeat with the remaining dry ingredients and continue to fold until smooth and free of lumps. Gently mix to keep the batter light and fluffy; be careful not to overmix. Set aside.
In a bowl, combine 6 egg whites, ¾ cup granulated sugar, and cream of tartar. Beat on medium-high speed until glossy and firm peaks form, about 7-8 minutes.
Fold ⅓ of the meringue into the wet ingredients. Then, carefully fold in the rest of the meringue until just combined and no white streaks are visible. Do not overmix to keep the cake light and airy.
Divide the batter between the pans and smooth the top with a spatula. Tap the pans gently on the counter a few times and run a small knife or a skewer through the batter to release remaining bubbles or air pockets.
Cool the cake completely in the pans, preferably on a wire rack. Once cooled, run an offset spatula or knife along the edges of the pan to loosen the cake. Then, carefully flip each cake onto a plate to remove it from the pan.
Using a long, serrated knife or a cake leveler, cut each cake horizontally into two equal halves, creating four layers in total. Reserve one layer to use as cake crumbs for decoration.
In a bowl, combine heavy cream, powdered sugar, and vanilla extract. Beat on medium-high speed until firm peaks form, about 3 minutes.
Crumble the reserved cake layer into fine pieces by hand or with a food processor. Place a baking sheet under the serving plate to catch crumbs. As you hold the cake, gently press the crumbs onto the sides, rotating for even coverage. Finally, sprinkle and press crumbs lightly on the top.
Refrigerate for a few hours before serving.