Grease and line 2 baking sheets with nonstick baking paper.
Whisk the egg whites in a thoroughly clean bowl until stiffly peaking. Whisk in the golden caster sugar 1 tablespoon at a time, then whisk in the light muscovado sugar, also 1 tablespoon at a time. Fold in the chocolate. Drop teaspoonfuls of the meringue mixture onto the baking sheets.
Bake in a preheated oven, 140°C (275°F), Gas Mark 1, for 1¼ hours, then turn off the heat and leave in the oven for another 30 minutes. Once cooled, gently peel away the paper from the meringues and store in paper cases in an airtight container for up to 3 days.