Preheat the oven to 180ºC (350°F/Gas 4) and grease a 12-hole muffin tin with butter.
Heat the chocolate and butter in a bain-marie or a heatproof bowl set over a pan of just-simmering water, until melted and shiny (don’t allow the bowl to touch the water or the chocolate will overheat). Remove from the heat and leave to cool slightly.
Whisk the eggs, sugar, coffee and rum together in a bowl for 3–4 minutes, until pale brown and moussy – the beaters should leave trails across the surface of the mix when lifted up.
Use a large metal spoon or plastic spatula to fold together the chocolate mixture and the egg mixture, employing a swiping motion through the mix rather than stirring. This may feel like it’s taking ages but it’s the best way to avoid knocking all the precious air out of the eggs.
Pour the mixture into the holes of the greased muffin tin, but don’t fill to the top; leave 2 mm (¹/8 in) or so for the puddings to rise, or they will spill over.
Slide the tin into the oven and bake for 13 minutes, or until the cakes have puffed up proudly. Remove from the oven and cool completely, in the tin.
As they cool, the centre of the cakes will sink (this is normal). Gently turn the cakes out of the tin, running a palette knife or spatula around the edges to loosen if necessary.
Whip the cream in a bowl until it forms soft peaks, then stir in the lime zest.
Just before serving, place a teaspoonful of dulce de leche into the little wells which have formed in the centre of each cake, then top each one with a dollop of the zesty cream. Serve straight away.