For the chocolate sauce in a small saucepan over medium heat combine 80ml water, 45gr sugar, the sifted cocoa powder, cook 2min stirring until uniform. Add the chopped dark chocolate, cook until completely melted and glossy.
Dissolve the cornstarch in 50ml whole milk, leave to rest.
For the ice cream base in a small saucepan over medium heat combine the leftover milk, heavy cream, 85gr sugar, a pinch of salt and 100gr Original Marshmallow Fluff or plain marshmallows and cook until fully dissolved. Add the cornstarch milk and cook constantly stirring until thickened and glossy smooth. Take off heat, cool to room temp and transfer into the fridge overnight.
Freeze in your ice cream maker according to the manufacturer's instructions adding the crushed shortbread cookies at the last couple of minutes of churning. Store in an airtight container alternating layers of ice cream and the chocolate sauce and store in the freezer until set.