1. Grease an 8 x 8-inch square baking tin with vegetable oil and line with a sling of baking paper. Grease the baking paper with a layer of oil too then set aside.
2. Combine the sugars, syrup, water and salt in a large pot and place on the stove over a medium heat. Leave to cook, stirring occasionally, until the mixture reaches 116C. If it looks like it’s going to boil over at any point just reduce the heat and the bubbles should subside.
3. Meanwhile, place the pumpkin puree into the bowl of a stand mixer with the whisk attachment fitted (or a large heatproof bowl if using a handheld electric whisk). Sprinkle over the powdered gelatine and add all of the spices. Stir together to form a thick paste then set aside.
4. Once the sugar mixture has reached 116C, remove it from the heat. Start the stand mixer on a low speed (or start your electric beaters on the slowest setting) and slowly start to pour in the hot syrup, being careful to pour it down the side of the bowl rather than onto the whisk. Once all the syrup is in the bowl, increase the speed to high and whisk until very thick, pale and fluffy.
5. Working quickly, pour and scrape the marshmallow mixture into the tin you prepared earlier, spreading it out into an even layer. Leave to set for at least 3 hours, or preferably overnight.
6. To cut the marshmallows, combine equal volumes of cornflour and icing sugar in a small bowl. Dust the top of the marshmallows with this mixture using a sieve. Flip the marshmallows out onto a clean work surface (or a large piece of baking paper) and peel away the greased layer of baking paper from the underside. Dust with more of the cornflour mixture. Use a large, sharp knife to cut the marshmallows – brush the knife with vegetable oil and dust with cornflour mixture to help prevent the marshmallows from sticking to the knife.
7. Roll each cut marshmallow in more cornflour mixture to ensure all sides are coated. You can toss the marshmallows, a few at a time, into a clean sieve and shake to remove excess cornflour coating. Store the marshmallows in an airtight container for up to 1 month.