Prep the pan. Use an ungreased 10-inch tube pan with a removable bottom. Do not butter or line it. The batter needs to grip the sides to climb and rise.

Make citrus sugar. In a small bowl, rub the orange zest into 1 cup of the granulated sugar with your fingertips until the sugar is fragrant and slightly moist. This releases the oils and boosts flavor.

Sift the dry mix. Sift cake flour, cocoa powder, and the remaining 1/2 cup granulated sugar together twice. Whisk in the salt. Set aside.

Whip the whites. In a clean, grease-free bowl, beat egg whites on medium speed until foamy. Add cream of tartar. Continue beating to soft peaks.

Add sugar gradually. With the mixer on medium, slowly rain in the orange sugar, a tablespoon at a time. Increase speed to medium-high and beat to glossy soft-to-medium peaks. The tip should curl slightly; avoid stiff, dry peaks.

Flavor the meringue. Beat in vanilla and orange juice just until combined. Don’t overmix.

Fold in the dry ingredients. Sift the flour-cocoa-sugar mixture over the meringue in 3 additions, gently folding with a large spatula between each addition. Use broad, light strokes, turning the bowl as you go to keep the batter airy.

Fill and smooth. Spoon the batter into the tube pan. Run a knife through the batter to pop large air pockets, then smooth the top.

Bake. Bake at 350°F (175°C) for 35–40 minutes, until the top springs back when lightly pressed and a skewer comes out clean.

Invert immediately. Remove from the oven and invert the pan onto its legs or over a bottle neck. Let it cool completely upside down, about 2 hours. This prevents collapse.

Release the cake. Run a thin knife around the edges and center tube to loosen. Lift out the insert and run the knife under the bottom to release. Transfer to a plate.

Finish and serve. Dust with powdered sugar or whisk the optional glaze and drizzle over the top. Add extra zest or chocolate shavings if you like. Slice with a serrated knife using a gentle sawing motion.