Nutrition per serving may change if servings are adjusted.
2 medium seedless oranges, scrubbed
3 cups water
3 cups sugar
3 cups sifted cake flour
1 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1 medium seedless orange, scrubbed and cut into eighths
2 cups sugar, divided
2 large eggs, separated
1 cup buttermilk
1/4 cup canola oil
1 teaspoon vanilla extract
2 large egg whites
Fresh mint sprigs, for garnish
To make candied oranges: Cut 2 oranges into paper-thin slices.
Mix water and 3 cups sugar in a wide pan. Bring to a simmer over low heat, stirring to dissolve sugar. Add the orange slices and poach, keeping the syrup at a bare simmer and ladling it over the oranges occasionally, until their skins are translucent, 1 1/2 to 2 hours (handle gently to avoid tearing). Carefully transfer the oranges to a sheet of wax paper; let cool. Reserve about 1/2 cup syrup.
To make cake: Meanwhile, preheat oven to 350 °F. Coat a 10-inch angel food cake pan with nonstick spray. Line the bottom with parchment paper.
Whisk flour, baking powder, baking soda and salt in a medium bowl. Set aside.
Pulse 1 orange in a food processor until finely chopped. Add 1 1/2 cups sugar; process until smooth. Add egg yolks, buttermilk, oil and vanilla; pulse to combine. Transfer to a large bowl.
Beat the 4 egg whites in a clean bowl with an electric mixer on medium-high speed until soft peaks form. Gradually sprinkle in the remaining 1/2 cup sugar, beating until stiff and glossy.
Add the dry ingredients to the orange mixture in 2 additions, folding with a whisk until just blended. Whisk one-quarter of the beaten whites into the batter. Fold in the remaining whites with a rubber spatula. Scrape batter into prepared pan and smooth the top. Tap the pan on the counter to release air bubbles.
Bake the cake until a skewer inserted in the center comes out clean, 55 to 65 minutes. Let cool in the pan on a wire rack for 10 minutes. Loosen edges and invert cake onto rack. Remove paper and let cool completely.
Place the cake on a platter and brush with the reserved 1/2 cup orange syrup. Arrange the candied orange slices on top and garnish with mint sprigs.
Make Ahead Tip: Cover and refrigerate the candied oranges (Steps 1-2) for up to 2 days.