Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, cream together the softened butter and sugar until smooth.
Beat in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the flour, black cocoa powder, and salt.
Gradually add the dry ingredients to the butter mixture, mixing until a stiff dough forms.
Scoop rounded tablespoons of dough, roll into balls, and place on the baking sheet.
Use your thumb or the back of a measuring spoon to press a deep well into the center of each cookie.
Bake for 10-12 minutes until set, then allow to cool completely on a wire rack.
In a chilled bowl, whisk the instant pudding mix with the heavy cream until stiff, mousse-like peaks form.
Pipe or spoon the chocolate mousse into the center of each cooled cookie well.
Garnish with chocolate shavings and chill before serving.