Grease a 20cm square tin with the oil, then add 40g of the cornflour to the tray, and shake the tray on all sides so it's lightly covered with cornflour. Let any excess cornflour sit on the base.
Add the gelatine leaves to a bowl of water and let it soften. Gently melt the chocolate in a bain-marie and once melted, take off the heat and stir occasionally.
Heat the sugar, liquid glucose and water in a small saucepan over a low heat until it dissolves, then increase the heat and let it bubble away until the mixture reaches 125°C on a sugar thermometer.
Meanwhile, add the egg whites to a clean bowl of a stand mixer and whisk until stiff peaks form.
Pour the sugar syrup into the mixing bowl, with the mixer still on, being careful not to pour any onto either the sides of the bowl or the whisk attachment. Once all the sugar syrup has been added, squeeze out the gelatin leaves with your hands to remove excess water and add them to the mixing bowl. Continue to whisk for another 10-15 minutes, until the bowl feels cool to the touch and the mixture is thick and glossy.
Pour the melted chocolate and sea salt into the marshmallow mixture and fold through 6 times, then transfer into the prepared brownie tray. Sprinkle with a final pinch of sea salt.
Leave to set for 5 hours or overnight in a cool spot, but not in the refrigerator. Mix the remaining cornflour with the icing sugar and cacao powder in a bowl, then scatter onto a clean work surface. Turn the marshmallow onto the surface and cut into 24 pieces with a large, sharp knife. Serve immediately or store in an airtight container for up to 3 days.