Place the egg yolk in a bowl and whisk through the water and cocoa powder until combined

Still whisking, slowly pour in the olive oil in a steady stream as if making a mayonnaise. Continue whisking until all the oil has been added and the mixture is glossy and emulsified

Create a bain-marie by placing a bowl over a pan of simmering water, ensuring the bottom of the bowl does not touch the surface of the water. Break the chocolate into chunks and add to the bowl, allowing to gently melt

Once the chocolate has melted completely, carefully remove the bowl from the heat. Add the butter and egg white and whisk using an electric hand whisk until smooth and shiny

Fold through the egg yolk mixture, mixing well until fully incorporated. Leave the mousse to rest for a couple of minutes, allowing the chocolate to set slightly

Transfer the mixture to an espuma gun with 2 charges. Reserve in the fridge if using as part of a larger dessert, or siphon mounds of mousse across small plates and dust liberally with cocoa powder to serve