In a medium mixing bowl, use a wooden spoon to beat the butter and sugars together until homogenous.
Add the egg into the bowl and beat into the mix, scraping down the bowl as you go.
Pour the melted chocolate into mixture and beat well with the wooden spoon.
Place a sieve on top of the bowl and add to it the flour, cornflour, cocoa powder, bicarbonate of soda and milk powder; sift these into the mixture and stir together until no lumps remain. Cover the bowl with cling film and place in the fridge until firm, 2-4 hours.
Evenly divide the dough into 8 balls of dough, about 50g each. Lightly flatten each ball of dough and place a frozen scoop of Nutella in the centre of each. Fold the sides of the cookie up and around the centre filling, enclosing it entirely. Repeat with the remaining cookies. Place in the freezer until frozen.
When ready to bake, preheat oven to 180°C. Place cookie/cookies on a tray lined with baking parchment and bake for 15-ish minutes, or until the edges are set and the centre appears a lil gooey. *see note for tips on perfectly circular cookies.
Finish the baked cookies with a sprinkle of flaky sea salt.