Preheat the oven to 200C/180C Fan/Gas 6. Line two baking trays with silicone mats or baking paper.
Heat the milk, butter, sugar and 125ml/4½fl oz water in a saucepan over a high heat until the butter has melted (do not boil).
Add the flour all in one go and beat firmly and quickly with a spatula or wooden spoon, keeping the pan over the heat. Cook the mixture for one minute, then transfer to the bowl of a food mixer and beat for a further 1-2 minutes, or until cooled slightly (you can do this by hand if preferred).
Add the eggs one at a time, beating well after each addition, until the mixture is smooth and glossy.
Transfer the pastry to a piping bag fitted with a large plain nozzle and pipe a large spiral, about 24cm/10in in diameter and 1cm/½in deep, onto one of the prepared baking trays.
Pipe small, evenly sized balls of the remaining choux pastry onto the second baking tray. Using a wet finger, gently smooth any pointed peaks on the top of the balls.
Bake the choux pastry in the oven for 20-25 minutes, or until golden-brown and cooked through. Set aside to cool slightly, then pierce the sides of the small buns once using a small, sharp knife to let the steam out. Do the same to the large pastry base.
For the filling, whisk the custard with the double cream until soft peaks form when the whisk is removed. Add the orange liqueur and whisk until firm peaks form when the whisk is removed. Spoon one-third of the mixture into a piping bag fitted with a small nozzle.
Pierce the flat bases of 16 of the small choux buns with a knife, insert the nozzle of the piping bag, and fill each with some of the cream mixture.
Heat the sugar in a frying pan over a high heat until it melts and starts to caramelise to a light golden-brown; swirl the pan, but don’t stir. (CAUTION: melted sugar is very hot. Take care not to burn yourself.) Working quickly, dip the tops of each filled choux bun into the caramel to coat, then return them to the tray until the caramel has set.
To serve, place the choux pastry base onto a serving plate. Cover the pastry base with quenelles of the remaining cream mixture. Arrange the caramel-topped choux buns at the edge of the pastry base, then fill the centre with the soft fruit.