Preheat oven to 180C. Whisk eggs, sugar and vanilla in an electric mixer until pale and tripled in volume (6-8 minutes). Sift over flour, fold in with a metal spoon, then fold in butter. Divide evenly among 2 greased, baking-paper-lined 25cm x 35cm baking trays, bake until light golden and centre springs back when pressed lightly with your fingertip (8-10 minutes). Cool completely, trim into four 7cm x 29cm rectangles, trim tops flush and set aside.
For white chocolate mousse, melt chocolate in a heatproof bowl over a saucepan of simmering water, stir until smooth, set aside. Bring 50ml cream to the simmer in a small saucepan. Squeeze excess water from gelatine, add to cream, remove from heat, stir to combine, then stir into chocolate. Whisk egg in a heatproof bowl over a saucepan of simmering water until thick and pale (1-2 minutes), fold through chocolate mixture and set aside. Whisk remaining cream until soft peaks form, fold through chocolate mixture and refrigerate until beginning to set (30 minutes).
For whipped mocha ganache, combine chocolate, cream and espresso in a heatproof bowl over a saucepan of simmering water, stirring until melted and smooth, set aside. Whisk yolks and sugar in a heatproof bowl over a saucepan of simmering water until thick and pale (3-4 minutes), fold through chocolate mixture then whisk in an electric mixer until cooled to room temperature (5-6 minutes).
Place a sponge rectangle in the base of a greased and baking-paper-lined 7cm x 29cm cake tin. Spread over half the whipped mocha ganache, smooth top. Top with a sponge rectangle, spread over white chocolate mousse to 1cm thick (there may be a little left over), smooth top. Repeat with another sponge rectangle and remaining whipped mocha ganache, top with remaining sponge rectangle, cover with plastic wrap and refrigerate until firm (3-4 hours).
Meanwhile, for mocha glaze, combine sugar and 60ml water in a saucepan and bring to the boil over medium-high heat, stirring to dissolve sugar and brushing sides of pan with a wet pastry brush to remove sugar particles. Cook until dark caramel in colour (4-5 minutes), remove from heat, add 100ml water (be careful, it will spit). Return to heat, add espresso and chocolate, stir until smooth and combined, then set aside to cool to room temperature.
Turn cake out of mould, thinly slice with a hot knife, serve immediately with mocha glaze.