Spray a large bundt pan (30 cm) with vegetable oil (wipe off excess with kitchen paper/towel).
Set the oven to 180 degrees Celsius (non-fan forced).
Melt the dark chocolate over a double boiler or microwave (taking care not to burn the chocolate). Set aside to cool.
In a stand mixer, beat the butter until fluffy, gradually adding the sugar.
Beat in the eggs one at a time.
Remove mixing bowl from the stand mixer and slowly mix in the melted dark chocolate with a spatula.
Mix in the rum and vanilla bean paste.
Sieve the flour, baking powder, baking soda, salt and cocoa powder into the wet mixture and mix thoroughly.
Add in the hot water, taking care to completely mix through.
Pour the mixture into the prepared bundt pan and bake for 45-55 minutes (or until wooden skewer comes out clean).
Let cool in the bundt pan for 10 minutes before turning over to a wire rack to cool.
Once cake is fully cooled down, dust on some icing sugar (I've recently discovered non-melting icing sugar and it works a treat for photography!)