For mango jelly, combine mango and 1 litre of water in a large saucepan, bring to a simmer over medium heat and cook for 30 minutes or until pulpy and fragrant. Purée using a hand-held blender or food processor, then strain through a fine sieve to yield 400ml, discarding pulp. Combine liquid with sugar in a clean saucepan and stir over medium heat until sugar dissolves. Bring to the boil, remove from heat and stir through lemon juice. Squeeze excess water from gelatine, add to pan and stir to dissolve. Stand until cool.
Meanwhile, preheat oven to 180C. Cream 140gm butter and 80gm brown sugar until pale and fluffy, add flour and coconut and stir to combine. Press out to a rough 1cm-thick rectangle on a baking paper-lined oven tray and refrigerate for 30 minutes. Bake for 12-15 minutes or until golden, then cool completely. Break up coconut biscuit and process using a food processor until finely ground. Melt remaining butter, add to food processor with remaining sugar and process to combine. Place ring of a 22cm-diameter springform cake pan (do not use base) on a baking paper-lined oven tray. Press coconut mixture into ring and refrigerate for 1 hour.
For mascarpone mousse, combine mascarpone and lemon rind in a bowl, folding gently (be careful not to split mascarpone). Heat milk in a small saucepan over low heat to just below simmering point. Squeeze excess water from gelatine, add to milk and stir to dissolve. Stand for 5 minutes to cool, then fold gently into mascarpone mixture. Whisk remaining ingredients in a heatproof bowl over a saucepan of simmering water for 5-7 minutes or until thick and pale, then carefully fold through mascarpone mixture. Spoon over crumb base, smooth top and refrigerate for 2 hours or until firm.
Pour mango jelly over mascarpone mixture and refrigerate overnight or until firm. To serve, remove cake pan ring and scatter cake with shaved coconut.