Preheat the oven to 110°C, fan 90°C, gas ¼. Line two baking sheets with greaseproof paper.
In a clean, grease-free bowl, whisk the egg whites until they form stiff peaks. Slowly add the caster sugar, tablespoon by tablespoon, whisking until it is all combined.
Divide the mixture between two bowls, then gently fold the cocoa powder into one and the jam into the other.
Drop large dollops of the mixtures onto the prepared baking sheets. You will get 3 meringues from each of the 2 mixtures.
Bake in the oven for 2 hours, then turn the oven off and leave in the warm oven for a further 1 hour before removing. Allow to cool completely before serving.
Cook's tip: use the leftover egg yolks to make a custard - or a simple glaze for pastries.