Preheat oven to 180C
Grease and flour six 180-millilitre muffin holes. (I used a Texas muffin tin). I recommend lining the base of each muffin hole with baking paper to prevent them sticking.
Combine butter, ½ cup of sugar and 2 teaspoons of zest in a food processor. Process until pale and fluffy.
Add eggs one at a time and process to combine.
Place the mixture into a bowl. Stir in flour and buttermilk then divide mixture evenly to make the six cakes.
Bake for 25-30 minutes or until cooked.
Meanwhile combine yuzu juice and remaining sugar in a small saucepan and stir over low heat until sugar is dissolved then simmer for five minutes until slightly thickened.
When the cakes come out of the oven rest them for 10 minutes then turn out and place upside-down into a small baking dish.
Pour half of the syrup over the warm cakes. (The cakes can be covered and refrigerated for up to 2 days at this stage.)
Reheat the remaining syrup with the extra rind and simmer until reduced and really syrupy.
To serve, reheat the covered cakes in the baking dish at 160C for around 20 minutes or until heated through. Place each cake onto a serving dish. Pour warm yuzu syrup over and accompany with a dollop of Greek yoghurt or thick cream