Preheat the oven to 180°C and line two 18 x 30cm trays with parchment.
Beat caster sugar and eggs until pale, then fold in flour and melted butter.
Bake the sponge for 10 minutes, cool, and cut out eight 7cm discs.
Place a disc in each ring, soak with liqueur, and brush with melted chocolate.
Stir soaked gelatin into hot coffee for the coffee mousse.
Whisk icing sugar and egg yolks, then fold in coffee and whipped cream.
Whisk egg whites to stiff peaks and fold into the coffee mousse mixture.
Half-fill rings with mousse, add another soaked sponge disc, and top with dark chocolate.
Melt chopped chocolate for the second mousse and beat in an egg yolk.
Whisk egg whites with sugar until stiff and glossy.
Fold egg whites into cooled chocolate and pipe the mixture onto the cakes.
Heat cream with coffee granules for the anglaise.
Whisk yolks, cornflour, and sugar, then cook with coffee cream until thickened.
Heat sauce ingredients with 75ml water until melted and smooth.
Remove rings using a blowtorch, dust with cocoa, and serve with sauce and custard.