Preheat your oven to 160C Fan/180C/325F. Brush the bundt pan with butter and refrigerate to harden the butter. After about 15 mins, take the bundt pan from the fridge and place a large spoonful of granulated sugar in the pan. Rotate the pan until there is an even coverage of sugar, tip away any excess sugar.
Sieve the ground almonds and plain flour into a large bowl and mix to combine.
Place the butter in the bowl of a stand mixer and beat with the paddle attachment until smooth.
Add the sugar and mix until fluffy, you may need to stop and scrape down the sides as needed.
Turn the mixer to a low speed and add a third of the eggs, mix until just combined, scrape down the bowl and add the remaining eggs.
The mixture will be really runny and fully mixed, be sure not to over mix the eggs. Add the flour a third at a time.
Scrape the bottom of the bowl to incorporate all of the dry ingredients, when fully mixed take 250g of the batter and stir in 75ml of the rum, mix until fully combined.
Add the batter and rum mix to the cake and stir well with a spatula, don't be alarmed if the finished mixture is not smooth.
Scrape the mixture into the prepared bundt tin, spreading the mixture round as evenly as possible. Tap the tin on the worktop to ensure it's distributed evenly and move the tin bake and forth to distribute the batter evenly.
Bake the cake for 1 hour, until golden brown and a skewer comes out clean. Set the pan on a cooling tray and leave for 10 minutes.
While the cake is baking, make the rum syrup by boiling the sugar and water together until all of the sugar has dissolved. Add 50ml of rum and leave to cool.
Turn the cake out onto the cake rack, placed over a baking tray or some greaseproof paper and leave to cool for 10 minutes.
Using a pastry brush, coat the cake evenly with the rum syrup and allow to cool completely.
To make the rum icing, sift the icing sugar into a bowl and add the water and dark rum, mix until the icing is smooth, place the bundt on a cake stand or plate and spoon or pour the glaze over the bundt.
The cake is better the next day when the ground almonds have had a chance to soak in the rum. In an airtight container this cake keeps well for 5 days.