Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, line a swiss roll tin or lipped baking tray with baking paper (grease it lightly to help it stick). The one I used was a Wilton tray that is 33.7 x 23.5 cm (13.25 x 9.25 inches)
Add the egg whites to the bowl of your electric stand mixer, or you can put them in a mixing bowl and use a hand held electric mixer. Whisk them until they are foamy and reach soft peaks stage – when you can tip the bowl and they don’t move (this takes about 3 minutes in my mixer on speed 6)
Keep the mixer running and add the caster sugar one spoonful at a time, then add the cornflour and lemon juice. Mix on a high speed for 3-4 minutes until thick and glossy, you want to be able to rub the meringue between your fingers and not be able to feel any sugar grains
Spread the meringue out into the tin, and bake for 10 minutes. Then, turn the oven down to 140C Fan/160C/300F/GasMark 3 and bake for a further 15 minutes, or until risen and just starting to colour
While it is baking you can make the blackberry coulis. Put the ingredients into a small pan, and heat on low. Mash the blackberries down and let the mixture cook for 5 minutes. Remove from the heat and push through a fine sieve to remove the seeds. Leave to cool before using
Before you take the roulade out of the oven, prepare a sheet of baking paper dusted with icing sugar
Remove the roulade from the oven and tip it out onto the icing sugar covered baking paper, remove the tin and the baking paper it was baked with, then leave to cool
Whip up the double cream with the icing sugar and vanilla extract until thick, an electric mixer is best for this
Spread most of the double cream over the roulade (save a little bit to decorate the top if you like), then add the blackberries all over and drizzle over about half of the blackberry coulis
Roll up the roulade, don’t worry about neatness or cracks – it's normal
Place onto a serving plate, drizzle with more coulis, dollop or pipe any remaining whipped cream down the centre, then add a few blackberries and some mint leaves if you like
Serve immediately, or refrigerate and eat within 2 days