Grease 5 x ramekin dishes with melted butter or cooking spray. Sift some cocoa powder on if desired.
Add the chocolate and butter to a microwave-safe bowl and melt in 30-second bursts. Stir and repeat until fully melted (you could also use the Bain Marie method).
Add the eggs to a large bowl, together with the sugar, vanilla and cinnamon. Mix well with an electric whisk or balloon whisk for around 1 minute.
Slowly add the chocolate and butter mix to the egg mixture and then sift the flour in. Fold everything together carefully until you have a smooth chocolaty batter.
Add equal portions of the mixture to the pre-prepared ramekins. Place the trivet in the instant pot and add 250ml of cold water. Place the ramekins on the trivet (if 5 won't fit put 4 in and balance the 5th in the middle).
Put the lid on the instant pot and set to high pressure for 10 minutes. Perform a quick release and remove the lid (be careful not to drip water from the lid onto the cakes).
Place the ramekins on a cooling tray and sift on some icing sugar if desired. Serve straight away.