1. Turn the cake tin upside down and sit it onto one of the flan cases. Press down to stamp out a circle. Repeat with the second flan case. With a long, sharp knife, carefully slice each one in half horizontally, giving you four thin sponge discs. Set aside the best- looking one for the top. Grease the sides of the cake tin with oil and line with parchment paper. Sit one of the sponge discs in the bottom.
2. To make the blackberry compote, place the blackberries in a small saucepan with the sugar. Crush roughly and cook over a medium heat for 1-2 minutes until soft and slightly thickened. Remove and leave to cool.
3. To make the marshmallow, pour 75ml of cold water into the bowl of a food mixer. Sprinkle in the gelatine and stir gently to ensure it’s all soaked. Leave for 5-10 minutes until all the water is absorbed.
4. Meanwhile, put the caster sugar, maple syrup and 100ml of water in a small heavy-based saucepan and stir over a low heat until the sugar dissolves. Then turn up the heat, bring to the boil and leave to bubble gently until it reaches 130°C on a sugar thermometer (if you don’t have one, you need the mixture to reach hard-ball stage – drop a tiny bit of mixture into a bowl of iced water and it should turn into a hard ball when rolled between your fingers). This takes 4-5 minutes and the syrup will become dark amber in color. Remove immediately from the heat and leave to cool for 1 minute.
5. Once the gelatine has soaked up all its water, turn on the food mixer to a low speed to break it up a bit. Then slowly and carefully trickle in the warm syrup, down the inside of the bowl rather than onto the whisk. Once all the syrup is added, whisk for 2-3 minutes until the mixture grows in volume and becomes stiff and meringue- like. Fold in the blackberry compote with a spatula, gently rippling it rather than completely blending.
6. Working quickly, pour one-third of the mixture over the sponge base in the tin, levelling it with the back of a spoon. Lay another sponge disc on top, followed by another third of mixture, again levelling smooth. Add a third disc and the remaining filling, and finish with the final (best-looking!) sponge disc on top. Cover with cling film and leave to set for about an hour until firm when pressed. The gateau can be prepared to this stage up to 24 hours in advance. It doesn’t need to be stored in the fridge, just keep it airtight.
7. When ready to serve, remove from the tin, peel off the paper, sit on a cake stand and dust the top evenly with icing sugar. Carefully heat the length of a thin metal skewer in the flame of a gas hob or blowtorch until searingly hot. Press the hot skewer onto the centre of the cake top to make a brand mark. (The smoke is nothing to worry about; it creates a bit of drama!) Continue to brand the top at 2.5 cm intervals either side of this middle line, wiping and reheating the skewer each time before pressing it down. Then turn the cake 45 degrees and repeat the spaced markings. This creates a pretty diamond pattern.
8. Finally, arrange the whole blackberries in a neat pile on the cake and add the mint sprig. Softly whip the cream (if using) and serve along with the blackberry purée. Cut the cake into eight wedges.