For the crab mayonnaise, place the egg yolks, vinegar and mustard into a food processor and blend until well combined.
With the motor running, slowly add the oils and blend until pale and creamy. Season with salt and freshly ground black pepper.
Place four tablespoons of the mayonnaise into a bowl, along with the crabmeat and chives and mix (the remainder of the mayonnaise, if there is any, can be covered and stored in the fridge for up to two days).
For the cucumber and scallops, with a vegetable peeler, peel eight ribbons from the cucumber and set to one side, then cut the remainder into small batons and place in a small bowl.
Place a spoonful of the crab mayonnaise on one end of each ribbon of cucumber and roll each ribbon up into a cylinder. Set aside.
Using a sharp knife, scrape the seeds of the vanilla pods into a small bowl, add the lime juice, half a teaspoon of sugar and the olive oil and whisk together until emulsified. Add salt and freshly ground black pepper, to taste, and adjust the quantity of sugar if necessary. Pour the dressing over the cucumber batons and toss to coat.
Heat a non-stick frying pan until hot and add the scallops. Fry on each side for one minute, then add the butter and cook for a further minute.
To serve, remove the cucumber batons from the dressing and divide them between four plates, placing them in the centre of each plate; top with two scallops each.
Place two of cucumber and crab rolls on top of the scallops on each plate, one upright, one on its side. Scatter the basil cress around the plate and spoon over a little of the cucumber dressing. Serve