Step 1

In a large mug (350 mL / 12.5 oz capacity or larger), stir together softened butter and boiling water until butter has melted. Stir in sugar until well combined. Stir in cocoa powder and self-raising flour until well combined.

Step 2

Press the 4 squares of chocolate down into the batter in the centre of the mug, making sure they are quite close together (this will give you a better molten lava centre).

Step 3

Microwave for 1 minute on full power until the cake has risen and there is no raw batter on the top of the cake. You can quickly stick a fork or spoon into the centre of the cake to see if it cooked through. Cook in further 10 seconds bursts, if needed (but be careful not to overcook). Serve with whipped cream or ice cream, if desired.