Preheat the oven to 180C/160C Fan/Gas 4. Generously butter a tarte tatin dish or similar of around 23–24cm/9–9½in diameter – you can also use a cast iron skillet as this is ovenproof.
Melt the sugar in a fairly wide saucepan, being careful not to stir but just swirling it around once in a while, until the sugar has melted and turned a light caramel colour. Remove from the heat and add the butter.
The liquid will bubble up, then subside, and you can then whisk the butter in. If the caramel splits, you can add a tablespoon of warm water and re-whisk – the sauce should re-emulsify.
Pour the caramel over the tarte tatin dish or skillet, then sprinkle in the cardamom and orange zest, if using. Arrange the bananas cut-side up over the caramel – they should be quite a snug fit.
Roll out the puff pastry into a round – or you can just roll it out and trim it to a round. Prick it lightly all over with a fork, then place on top of the bananas, tucking the edges in as you go. Bake for between 40–45 minutes, or until the pastry is a rich golden-brown. It may be slightly risen.
For the custard, heat the milk and cream together until just below boiling point. Remove from the heat.
Whisk the egg yolks, sugar, cocoa powder and cornflour in a bowl together, then pour in the milk and cream mixture into the bowl from a height in a steady steam, whisking as you go. Return to the saucepan (rinse it out first), then cook on a low heat, stirring constantly, until the custard thickens.
Serve slices of the tarte tatin with the chocolate custard on the side.