Preheat oven to 165C. Whisk eggs and sugar in a heatproof bowl placed over a saucepan of simmering water until mixture reaches 40C. Transfer to an electric mixer and whisk until pale and tripled in size (5-10 minutes). Combine flour and cocoa, then sift over egg mixture in batches, folding between additions until just incorporated. Fold in melted butter, then pour into a buttered and floured 20cm x 30cm retractable-base lamington pan (see note). Bake until golden and cake springs back when lightly pressed (20-25 minutes). Cool on a wire rack, turn out and set aside. Line pan with plastic wrap and set aside.
Meanwhile, for cherry syrup, combine sugar and 500ml water in a saucepan and stir over medium heat until sugar dissolves. Bring to the boil, add cherries, bring to boil again, reduce heat and simmer until tender (3-5 minutes). Remove from heat, set aside.
For cherry mousse, heat cherries, sugar and ginger over low heat until liquid seeps from cherries (5-10 minutes). Remove 2 cups cherries with a slotted spoon (reserve), then strain cherry liquid through a fine sieve (you should have 430ml). Transfer 180ml cherry syrup to a small saucepan and heat over medium heat until simmering (1-2 minutes). Squeeze excess liquid from gelatine, add to pan, stir to dissolve, then transfer to a bowl placed over ice. Leave to set (5-10 minutes).Meanwhile, whisk cream to stiff peaks and, when cherry mixture is almost set, fold through cream a third at a time. Set aside.
Cut cake horizontally into thirds. Line lamington pan with plastic wrap, with 30cm overhang either side. Place one cake layer into pan, brush liberally with cherry syrup and sherry, scatter over half the reserved cherries, then evenly spread over half the mousse with a palette knife. Place another layer of cake on top and repeat with remaining cherries and mousse. Place last cake layer on top, then cover with plastic wrap, weight down and refrigerate until set (6 hours-overnight). Dust with cocoa before serving.
Note Retractable-base lamington pans are available from Iced Affair.