Crush the cardamom pods with a pestle and mortar, then put the seeds and the husks in a medium pan. Pour in the cream and sugar, stir, then heat gently until the cream begins to simmer. Remove the pan from the heat and leave to infuse for 30 mins.
Meanwhile, finely chop the white chocolate in a food processor, then tip into a medium bowl. Repeat with the cacao, tip this into another bowl and add the butter. Season each chocolate with a pinch of salt.
Heat oven to 180C/160C fan/gas 4 and lightly grease a 23cm fluted tart tin or springform tin. Without cleaning the processor, tip in the digestives and pulse to fine crumbs. Pulse in the melted butter and sugar until the mixture looks like wet sand, then press firmly into the base of the tin. Slide it onto a baking sheet, bake for 10 mins, then cool completely.
When the base is cold, reheat the infused cream to a simmer. Strain half the cream on top of the white chocolate and stir. Add the milk to what is left in the pan, reheat, then strain this over the chopped cacao and butter, and stir. It will take a few mins before each chocolate melts completely.
Pour all but about 3 tbsp of the cacao ganache on top of the biscuit base and chill for 15 mins or until just set. Now flood the top with the white chocolate ganache. Finish with the reserved cacao mix,
dropping 6 dollops on top like a clock face, and one in the middle. Use a skewer to swirl the colours together. Chill for at least 4 hrs or ideally overnight to let the ganache set firm, then bring it out of the fridge for 30 mins before serving.