Preheat oven to 150β or 300F
Separate egg whites carefully into a large mixing bowl (see tips below) it's the most taxing bit of the recipe but it's kinda like THE most important bit to get right. Not even the titchiest bit of yolk can get in those whites, Susan pay attention.
Whisk those whites until really frothy with an electric whisk or free standing mixer.
Now the fun bit, add the brown sugar a tablespoon at a time as you whisk. Brown sugar is a clumpy beast so I use my clean fingers to scatter it in there, a generous pinch at a time (tbsp ish)
Keep whisking until all the brown sugar is in there and it is thick, glossy and smooth. It needs to be super stiff, so much so that it just won't take any shizz from nobody, least of all gravity.
Line a large flat baking tray with baking paper. I use a 8" cake tin to draw around onto the baking paper for a cute template otherwise I might end up with a rhombus.
Dollop that mixture onto your baking tray.
Use a flat spatula to spread out into a circle shape and raise up the outer edge like a crown, so that the middle is lower than those edges.
Turn the oven down to 140β. - 280β. Bake for about an hour. It will kinda go craggy, split a little around the outside and maybe drop in the middle somewhat, but it does not diminish it's beauty, in fact I rather think it adds to it's kerb appeal, think Daniel Craig rather than Timothee Chalamet.
Turn off the oven and let it go cold in there.
Fill with softly whipped cream. Watch your cream doesn't overwhip and lose it's floppiness. Something that escapes me sometimes very annoyingly, as the dolloping of softly whipped cream onto a pavlova is a truly beautiful thing to be a part of.
Top with blackberries and pour over your coulis, if using, when you are ready to serve. If you do it before you are ready to plate it up then the coulis will bleed a little into the cream. The alternative is to serve it in a jug on the table for people to help themselves. Make ahead city right there. Lovvvvve it.
slice with a large serving spatula and serve to squeals followed by silence....