Add espresso powder to heavy cream and whisk in large bowl until soft peaks form.
Gently fold hazelnut spread into the heavy cream. (If the hazelnut spread is particularly thick, warm it or add some of the heavy cream to it first so that it will incorporate better.)
Fill ramekin or other serving dish with the mousse, cover, and chill for an hour.
Garnish with a fresh raspberry and a sprinkle of Maldon sea salt.
Notes
If using small 6 ounce ramekins, this recipe will fill about three and half of them.