Before starting be sure whites do not have any trace of yok inside and that the bowl and whisks are clean, dry and with no trace of grease. In those cases the meringue won't come up.
Start beating egg whites, cold, alone for few seconds: they must be foamy.Now add the caster sugar mixed well with soluble coffee powder and beat for about 10 minutes.You must end with a shiny, hard meringue, it must form stiff peaks, you may need some minutes more. Now add sifted icing sugar, mixing with a spatula, no more whisking.
Now, using a sac a poche, form meringues the shape you want on a baking tray lined with parchment paper.Try to keep them the same size to let them bake evenly.Put them immediately in the oven ( no fan), already heated at 210°F / 100°C. Bake for 20 minutes then low the oven to 194°F/90°C nd bake for 1 hour and half.
Now open the oven a little, just a slot ( if your oven doesn't stay that open alone use a wooden spoon to keep the door ajar).Bake for 1 more hour. Now turn off the oven but let the meringue stay inside with the ajar door for 5-6 hours or overnight.
Store them in a dry place, a tin box is perfect.