Preheat the oven to 170C fan. Grease and line the bottom of a 20cm loose bottomed cake tin.
In a bain-marie melt the butter and chocolate together until smooth. In the bowl of a freestanding mixer beat the sugar and eggs together until very light and fluffy (roughly 3 mins).
Gently fold the chocolate mixture with the egg mixture and rum, if using, until uniform in texture and colour.
Gently pour into the prepared tin and bake for 30-35 minutes until set around the edges but still with a slight wobble in the middle.
Run a sharp thin knife around the tin when it comes out of the oven and allow to cool completely in the tin.
Once cool cover the tin to seal and place in the fridge for 6 hours or ideally overnight.
When ready to serve remove the cake from the tin and slice using a hot knife (boiling water or a gas flame are best for this) to achieve a clean cut.
Serve with Mackie’s Traditional, Chocolate or Madagascan Vanilla ice cream or a big dollop of crème fraiche.
Store in the fridge for up to 3 days.