Set a small to medium stainless steel bowl in a large bowl full of ice.
Add the oat milk, chopped chocolate, vanilla and espresso powder to a saucepan and place over a low heat, whisking until everything is melted and fully homogenous.
Pour the chocolate mix into the prepared bowl and whip vigorously until the mousse has thickened and lightened in colour, around 2 to 3 minutes (I like to do this by hand for greater control, but you could use an electric hand mixer instead, in which case it will take even less time). Transfer to individual serving coupes or glasses and place aside until ready to serve.
