Preheat the oven to 180C/350F/Gas 4.
For the pudding, place the butter and caster sugar into a food processor and pulse until smooth and creamy. Crack in the egg, add the flour and cocoa powder and, with the motor running, add enough milk to form a batter with a dropping consistency.
Spoon the pudding mixture into two buttered ramekins and place onto a baking tray in the oven to bake for 6-8 minutes, or until risen and springy to the touch.
For the guavas, melt the butter in a frying pan and fry the chopped guavas, sugar and lime juice for 4-5 minutes, or until the guavas are tender and caramelised.
For the sauce, place all of the sauce ingredients into a small pan and heat, stirring frequently, until the sugar has dissolved and the mixture reaches a simmer.
To serve, turn out the puddings onto serving plates, spoon over the guavas and drizzle over the chocolate sauce.