Begin by lightly greasing the base and sides of a 20cm square baking tin with a small quantity of oil. Neatly line the tin with baking parchment (non-stick)
Sift the icing sugar and cornflour together and use approximately ¼ to lightly dust the baking parchment
Put the gelatin leaves into a bowl of cold water and leave to soften (5-10 minutes)
Tip the egg white, 1 tablespoon of the caster sugar and the salt into a large bowl
Pour the rest of the caster sugar and the black treacle into a medium-sized heavy-based saucepan. Over a medium heat, allow the sugar to dissolve, stirring occasionally. Bring to the boil and continue to cook until the syrup reaches 120°C/ 248°F on a sugar thermometer - I used my Thermapen Professional As the syrup approaches 115°C/ 240°F, whisk the egg whites with electric beaters until stiff
As soon as the syrup reaches the specified temperature, take the pan off the heat. Drain the gelatin, add to the syrup and stir until completely dissolved
With the electric whisk running, steadily pour the syrup into the egg whites, taking care NOT to pour it onto the whisk. It will take around 1 minute to pour the syrup in at a steady pace
Once the syrup has been added, continue to whisk for 5 minutes
Add the pumpkin spice and whisk for a further 5 minutes. The marshmallow should be thick, glossy and stiff - it should leave a definite trail that does not melt away after 30 seconds.
If this consistency has not been reached whisk for a few more minutes and check again Once the correct consistency has been reached, use a rubber spatula to spoon the marshmallow into the prepared tin and level the top with a palette knife
Leave to set in a cool place for 2 hours, then cover with clingfilm and leave for 4 more hours
Dust a large board with the remaining icing sugar & cornflour mix. Tip the set marshmallow onto this board and peel off the baking parchment. Use a sharp knife to cut it into squares and dust all sides in the sugar & cornflour
Leave to 'dry out' for a further hour, then store in an airtight container for up to 5 days