The cupcakes can be stored for 3 days at room temperature.
The buttercream can be refrigerated for upto a week. Bring to room temperature and beat again for desired consistency.
For stronger coffee flavour , add more instant coffee powder or make a stronger decoction.
If you don't have castor or super fine sugar , pulse your regular sugar in the blender or processor once. You could also use granulated sugar but will need to beat the buttercream more until you cant feel any grains.
All ovens work different , if your cupcakes have cracked domes, try baking at 160 C for a few minutes longer!