Heat a convection oven to 350 degrees F or a conventional oven to 375 degrees F.
Cover the bottoms of six 3-inch ring molds with aluminum foil, folding the excess foil up and around the outside of the ring molds to create a bottom. Alternatively, you can use 6 similar-size ramekins.
In a double boiler or in a heatproof bowl set over, but not touching, simmering water, melt the butter and chocolate together, stirring occasionally.
In a stand mixer fitted with the whisk attachment, whisk the eggs and sugar on medium speed until doubled in volume, 10 to 15 minutes. Turn off the mixer, add the cocoa powder, and mix on low speed until blended. Using a large rubber spatula, fold in the melted butter and chocolate until just combined.
Fill the ring molds about three-quarters full. Bake until the cakes look set but are still a little wiggly in the center, 8 to 10 minutes.
Let the cakes cool, then remove from the molds. (The cakes may be made earlier in the day; let cool completely, then cover and keep at room temperature.)
To serve, reheat the cakes briefly in a warm oven and serve with orange gelato on the side.