Preheat oven to 350°F (180°C). Spray 2 (9-inch) round cake pans with baking spray with flour. Line bottom of pans with parchment paper; lightly spray parchment.

In a medium bowl, sift together flour, 1½ cups (300 grams) sugar, baking powder, salt, and baking soda.

In another medium bowl, whisk together melted butter, oil, and vanilla paste or extract until well combined. Whisk in buttermilk and egg yolks until well combined.

In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites and cream of tartar at medium-low speed until foamy. Increase mixer speed to medium-high; gradually add remaining ¼ cup (50 grams) sugar, beating until soft peaks form, about 1 minute. Increase mixer speed to high, and beat until stiff, shiny peaks form, about 1 minute. (Do not overbeat.) Transfer egg white mixture to another medium bowl; return bowl to stand mixer.

Switch to the paddle attachment. Add flour mixture to mixer bowl; with mixer on low speed, gradually add butter mixture, beating just until combined, about 15 seconds; scrape bottom and sides of bowl and paddle. Beat at medium speed until thick, smooth, and well combined, about 30 seconds. Fold in one-third of egg white mixture until almost combined. Gently fold in remaining egg white mixture until no white streaks remain. Divide batter between prepared pans, smoothing with an offset spatula. Tap pans on a kitchen towel-lined counter 6 to 8 times to release air bubbles.

Bake until edges are lightly browned and a wooden pick inserted in center comes out clean, about 20 minutes. Let cool in pans on a wire rack for 10 minutes. Remove from pans, gently remove parchment, and let cool completely on a wire rack.

Level cooled cake layers, if desired. Place 1 cake layer on a serving plate.

Transfer 1½ cups (265 grams) Whipped Cream Frosting to a pastry bag fitted with a large round piping tip. Pipe a border of frosting around edge of cake layer. Spread half of Blueberry Filling within frosting border. Pipe remaining frosting in bag onto filling; gently spread smooth with an offset spatula. Top with remaining cake layer. Spread remaining frosting on top and sides of cake, creating a slight border on top of cake. Spoon remaining Blueberry Filling on top of cake within border. Loosely cover and refrigerate for at least 4 hours before serving or up to overnight. Just before serving, garnish with blueberries and flowers, if desired. Cover and refrigerate for up to 4 days.