Melt the butter over low heat and grease a 16 cm diameter charlotte mold.
Whisk the egg yolks with the caster sugar and vanilla sugar until pale and creamy.
Lightly dip the spoon biscuits in the coffee and line the sides of the mold, domed side facing outward.
Soak the gelatin sheets in cold water, squeeze out the excess, and dissolve in 5 cl boiling water.
In a bowl, whisk the mascarpone into the egg mixture, then add the dissolved gelatin. Pour into the lined mold, top with a layer of dipped biscuits, cover with plastic wrap, and refrigerate until set.
To unmold, dip the mold briefly in hot water. Place a serving plate on top and invert. Dust the top generously with cocoa powder using a fine sieve.
Serve with segments of freshly peeled oranges.
