Preheat your oven to 375°F (190°C). In a bowl, sift together 1 cup of cake flour and 3/4 cup of sugar. In a separate bowl, whip 12 egg whites with a pinch of salt and 1/4 teaspoon of cream of tartar until soft peaks form. Gradually fold in the flour mixture, being careful not to deflate the batter. Pour the batter into an ungreased tube pan and bake for 40-45 minutes until lightly touched and springs back. Cool upside down before removing from pan.

In a double boiler, combine 1 cup of sugar, 3 large eggs, 1 tablespoon of flour, 1/2 cup of fresh orange juice, and the zest of one orange. Whisk continuously over simmering water until thickened, about 10 minutes. Remove from heat and let cool. Whip 1 cup of heavy cream until stiff peaks form, then gently fold into the cooled orange mixture.

In a large mixing bowl, beat 1 cup of softened unsalted butter until fluffy and pale, about 2-3 minutes. Gradually mix in 3-4 cups of powdered sugar, then add 1 teaspoon of vanilla extract and the zest and juice of one orange. Beat until smooth and creamy.

Slice cool angel food cake into thirds. Spread a layer of orange cream filling on the bottom layer, then repeat with the second layer. Place the final layer on top and smooth remaining orange cream filling around the edges.

Frost the assembled cake with orange buttercream, spreading evenly on the top and sides. Use a spatula to create swirls or designs. Garnish with fresh orange slices or a sprinkle of orange zest.

Refrigerate the cake for at least 1-2 hours before serving, allowing the filling to set and frosted layers to adhere.