Preheat your oven to 375°F. Sift together the cake flour and 3/4 cup of sugar in a bowl. In a separate large mixing bowl, beat the egg whites with cream of tartar until soft peaks form. Gradually add the remaining sugar, continuing to beat until stiff peaks form. Gently fold in the sifted flour mixture using a spatula.

Pour the angel food cake batter into a 10-inch tube pan, smoothing the top with a spatula. Bake in the preheated oven for 40-45 minutes, or until the cake is lightly golden and springs back when touched. Invert the pan to cool completely.

In a double boiler, combine sugar, eggs, all-purpose flour, orange juice, and zest. Cook over simmering water, whisking constantly until the mixture thickens, about 8-10 minutes. Remove from heat and cool before folding in the chilled heavy whipping cream.

Beat the softened unsalted butter until fluffy. Gradually add powdered sugar until smooth. Mix in vanilla extract, fresh orange juice, and zest until creamy.

Remove the cooled cake from the tube pan and slice it into thirds. Spread orange cream filling on the first layer, add the second layer, repeat filling, and top with the third layer. Frost with the orange buttercream.