Pour the blackberries into a medium sized pan and heat gently with a splash of water to aid the breaking down of the berries. Once the berries have completely softened then remove them from the heat and pour into a sieve. Push the berries through, the best aid for this I think is a silicone spatula, so that all the seeds are extracted from the fruit pulp. Discard the seeds (or save to make a fruit alcohol infusion as explained above) and return the pureed blackberries back into the pan.
Add the icing sugar and liquid glucose to the blackberry puree and bring to the boil. Remove from the heat then leave to cool before covering and refrigerating overnight.
Meanwhile pour the brown sugar into another medium sized saucepan and add 300ml of water. Heat gently so that the sugar completely dissolves into the water. Then bring to the boil and carrying on boiling until it reaches a very thick and syrupy consistency, it should reach 112°C on a sugar thermometer and can take about 20 minutes. You must keep your eye on the saucepan at all times so that it doesn’t bubble over.
In a large mixing bowl whisk up the egg whites until stiff, then drizzle in the sugar syrup in a slow steady steam whilst continuing whisking. The egg whites will turn beautifully glossy.
In a separate bowl lightly whip the double cream then fold into the egg whites until they are fully incorporated. Cover the bowl with cling film and place in the fridge overnight.
The next day pour the egg white and cream mixture into your ice cream machine and churn for about 15 minutes or until the ice cream has reached a thick milkshake consistency.
Finally whilst the mixture is still churning drizzle in the blackberry puree until it has rippled evenly throughout the ice cream. Turn off the ice cream machine then decant the ice cream into tubs and freeze overnight to reach the correct set.