For shortcrust pastry, process flour and sugar in a food processor to combine, add butter and pulse until mixture resembles fine crumbs. Add 2 tbsp chilled water and process until mixture just comes together. Turn onto a lightly floured surface, form into a disc, cover with plastic wrap and refrigerate for 1 hour.
Preheat oven to 190C. Roll out pastry to 5mm thick on a lightly floured surface, line a 26cm-diameter tart tin and refrigerate for 1 hour. Blind bake for 20 minutes (see note), then remove paper and weights and bake until golden (8-10 minutes). Cool in tin on a wire rack.
For caramel, combine sugar and 250ml water in a heavy-based saucepan and stir over low heat until sugar dissolves. Increase heat to high and cook, brushing down sides of pan with a wet pastry brush if sugar crystals form, until mixture is deep golden (10-15 minutes). Remove from heat, add cream and butter and stir to combine (be careful as mixture will spit). Pour into pastry base and refrigerate until firm (1-2 hours).
Meanwhile, for chocolate mousse, melt chocolate in a heatproof bowl over a pan of simmering water, remove from heat, cool to room temperature, add yolks, stir to combine and set aside. In a separate bowl, whisk cream until soft peaks form and set aside. Whisk eggwhites in the bowl of an electric mixer until soft peaks form, gradually add sugar and continue whisking until combined. Fold a third of the eggwhite mixture into chocolate mixture, then fold in a third of the cream. Repeat with remaining eggwhite and cream until combined, spoon over cooled caramel, smooth top flush with tart edges and refrigerate until firm (2-3 hours).
Combine chocolate and cream in a small saucepan, stir over low heat until chocolate melts, remove from heat, add butter and stir to combine. Cool to room temperature, then spread in an even layer over chocolate mousse. Refrigerate until set (30-40 minutes). Serve tart with whipped cream to the side.
Note For blind baking, line a pastry-lined tart tin with baking paper, then fill with weights (ceramic, rice and dried beans work best).