Method

1. Prepare a 20cm/8in square baking tin 4cm/1½in deep by lining it with baking parchment. Using a sieve, shake equal amounts of icing sugar and cornflour over the base until the parchment is lightly dusted.

2. To make the marshmallow, put the sugar, glucose and 200ml/7fl oz of water into a heavy-bottomed pan and stir. Bring the liquid to a gentle boil and continue cooking for about 15 minutes until the mixture reaches 127C/260F on a sugar thermometer. (CAUTION: boiling sugar is extremely hot. Handle very carefully. Use a deep pan to avoid bubbling over.)

3. While you are waiting, sprinkle the gelatine over 100ml/3½fl oz of hot water (following the instructions on the packet), and check to make sure it has dissolved properly. If using gelatine leaves, soak them in cold water for 10 minutes and then squeeze out any excess water before using.

4. When the syrup reaches the correct temperature, remove the pan from the heat and remove the thermometer. Pour the dissolved gelatine into the syrup. Be careful as it will bubble and spit a little and rise to the top of the pan. Caution: it is extremely hot.

5. Using a freestanding mixer or electric whisk, whisk the egg whites in a large mixing bowl until stiff peaks form when the whisk is removed. Still whisking, slowly add the syrup mixture to the eggs (do not let the syrup touch the beaters as it may splash). With the mixer or whisk on a fast setting, whisk for a further 15-20 minutes, or until the mixture is thick, shiny and holding its shape reasonably well on the whisk.

6. Meanwhile, to make the raspberry coulis, place roughly half the fresh raspberries in a pan with the caster sugar (set the other half aside for decoration later). Cook on a medium heat for around 10 minutes, stirring until it is a lovely ruby-red mush. Strain the mixture into a clean bowl, pressing the pulp through the sieve using the back of a spoon. Leave to cool.

7. Once you’ve finished whisking the marshmallow mixture, use a spoon to fold the raspberry coulis gently through the marshmallow mixture to create a rippled effect.

8. Spoon half the mallow mixture into the dusted tin and place half the reserved raspberries over the top. Spoon the remainder of the mallow mix over the top of the fresh raspberries, smoothing the top with a palette knife, if necessary. Scatter the remaining raspberries over the top.

9. Leave the mallow for about two hours to set.

10. Dust another piece of parchment with a mix of equal parts icing sugar and cornflour.

11. Turn the mallow out onto the dusted parchment. Cut it into squares using a knife dipped in hot boiled water. Dust the cut edges with more icing sugar and cornflower and transfer to a wire rack. Leave to dry and cool for another 30 minutes if serving as a gooey dessert, or 2-3 hours if cutting into squares.