Set the sugar for the roulade on a small pan, an 8 inch square cake pan or a quarter sheet baking pan work well. Place it in the oven on the middle rack and set the oven to preheat to 275 F (it’s important to double check the temperature of your oven, I suggest using an additional thermometer).
While the oven preheats, place the egg whites in the bowl of a stand mixer - it should be clean and free from any grease or oils. Affix the whisk attachment and begin to whisk on low, working up to medium speed. Whisk until all the egg whites are foamy, even at the bottom of the bowl.
Once the sugar is hot and the egg whites are done with their initial whisk, spoon the sugar into the bowl while the mixer is on and whisking: one spoonful at a time, waiting 10 seconds before adding another.
Once all the sugar is in, stop the mixer and scrape the sides of the bowl with a rubber spatula to ensure none of the sugar is stuck to the edge of the bowl. Continue whisking until the meringue is very close to stiff peaks, it should be thick, creamy looking, and when you lift the whisk the meringue shouldn’t droop too much. Add the salt and vanilla and whisk.
Note: you can also use the swiss meringue method. See above.
Once you have stiff peaks, add the starch and vinegar and whisk to just combine. Don’t over do this step.
Grease a jelly roll pan (10x15 inches) and place a sheet of parchment over it - the parchment should have overhang on all sides so the meringue does not touch the pan.
Spread the meringue into the pan in an even layer, be gentle with it but swift. Swoops are pretty at the top but avoid leaving any part of the meringue that will be considerably thinner as these will be the points where it is more likely to crack later on.
Bake the meringue for about 22 minutes, until it has puffed up, is matte and firm to touch. Take it out of the oven and set on the counter to cool for 5 minutes.
Over the meringue lay a sheet of parchment paper and over the parchment, a flour sack or tea towel. Carefully (with oven mitts) flip the meringue upside down so it sits on the fresh piece of parchment. Lift the pan away and peel away the parchment that the meringue baked on. Dust the now exposed, bottom of the meringue with powdered sugar.
Using the tea towel/parchment, slowly roll the meringue from the short side (you’ll have a short but fat meringue roulade) until it’s in a coil. It will probably crack a bit - don’t worry about it. Set aside to cool for at least an hour.
Make the ganache: chop the chocolate finely and set it in a heatproof bowl. Warm the heavy cream until it's steaming and bubbling only at the sides (don't let it boil), pour it over the chocolate and stir quickly. Leave it for 30 seconds then stir until smooth and all the chocolate has melted. I usually add a pinch of salt and a dash of vanilla while stirring. Once smooth, set in a cool area to cool (you can use the fridge to speed this up but you'll want to check on it frequently) and thicken. When it's thick like a nut butter, use a hand mixer to beat it until it's a few shades lighter and fluffy.
Carefully and gently unroll the roulade. Spread the ganache over it in an even layer, with swift but gentle swipes of an offset spatula. Spread the whipped cream on top and dot with berries (the smaller the berries the better). Roll up the roulade again, it's now ready to serve.
