To assemble the sundae:
Swirl some chocolate sauce around the base of a coupe glass.
Add a scoop of banana ice cream (see recipe below), then a few dried banana chips, followed by a scoop of chocolate ice cream and a few more dried banana chips.
A few mini-marshmallows are nice too, as they add another texture.
Pipe or spoon some Chantilly cream over the top.
Top with 5 chocolate mocha beans, drizzle with chocolate sauce and walnut sauce, then add some more Chantilly cream.
At Fortnum’s, we serve this with two of our supersized, ultra-chocolatey Chocolossus biscuits on the side
Note: If you cant get any banana ice cream, you can make a very quick and easy one by freezing 4 chopped ripe bananas for at least 2 hours, until completely frozen, then blitzing them in a food processor or blender with 2 tablespoons of honey and the zest and juice of 1 lime.
Adapted from Fortnum & Mason: The Cook Book by Tom Parker Bowles (Fourth Estate, 2016)