Preheat the oven to 200°C / 180°C fan / Gas 6.
To make the meringues, line a baking tray with parchment and pour over 250g of the caster sugar. Pop in the oven for 5-6 minutes, until the sugar is very hot.
Whilst the sugar is heating, whip the egg whites until they form stiff peaks. As soon as you take the sugar out of the oven, lower the oven temperature to 120°C / 100°C fan / gas ½. Carefully add the hot sugar in spoonfuls to the egg whites, making sure that you beat the whites back to stiff peak after each addition. Once all of the sugar has been incorporated, continue to beat until you have a stiff and glossy meringue. If using the strawberry powder, add it to the meringue and gently fold it through a couple of times to create a marbled effect.
Line two baking sheets with parchment and drop 8 dollops of meringue between the two. Use a spoon to swirl into a sort of nest, then bake in the oven for 1 hour, until crisp and set. They will still have a good chew within. Remove from the oven and set aside to cool completely.
Whilst the meringues are cooling, tip the sliced strawberries into a bowl and add the elderflower heads, 65g of the sugar and a tiny pinch of salt. Stir carefully to coat everything in the sugar, then set aside to macerate for 30 minutes.
Whip the mascarpone with the remaining sugar and the vanilla bean paste until light and fluffy.
Discard the elderflower heads and serve the meringues piled with the whipped mascarpone and the juicy strawberries.