Try this Ciambellone marmorizzato recipe, or contribute your own.
The structure of this tall ring cake relies entirely on mechanical aeration. Whip the whole eggs and granulated sugar with a stand mixer fitted with the whisk attachment for a full 10 minutes at medium-high speed. The mixture must triple in volume, turn pale yellow, and leave a thick ribbon on the surface when the beaters are lifted. Folding dry ingredients into this foam requires a gentle hand. Use a wide silicone spatula and cut down through the center, sweep along the bottom of the bowl, and fold the batter over itself while rotating the vessel. Vigorous stirring pops the trapped air bubbles, resulting in a dense, heavy crumb instead of a light sponge.
Liquid ingredients must be introduced gradually to prevent the whipped eggs from collapsing. Pour the whole milk down the inside edge of the mixing bowl in a thin, steady stream while the mixer runs on its lowest setting. Potato starch lightens the heavy texture of standard pastry flour, but it absorbs moisture quickly. Sift the potato starch, all-purpose flour, baking powder, and salt together twice before incorporation to eliminate lumps and distribute the leavening agent evenly. Greasing the 28-centimeter Bundt pan thoroughly with solid butter and dusting it with extra flour ensures the delicate ring releases cleanly after baking.
Dividing the batter properly creates distinct chocolate streaks without muddying the entire cake. Pour three-quarters of the plain batter into the prepared ring mold first. Whisk the unsweetened cocoa powder thoroughly into the remaining quarter of the batter until no dry pockets remain. Spoon the chocolate batter directly over the plain base, then drag a butter knife or skewer through the layers in a zigzag motion to distribute the dark streaks. Bake at 180 degrees Celsius for 45 minutes on the middle rack. Insert a wooden skewer into the thickest part of the ring; it must emerge completely clean without wet crumbs clinging to it before removal from the oven.