Roast the rhubarb by washing it, cutting into small pieces (5-6 cm in length) and sprinkling over 50 g caster sugar, then roasting for 15 minutes in a foil-covered dish. Then remove the foil and roast for 5 minutes more. Leave to cool.
Cream together the butter and sugar until light and fluffy.
Add the vanilla bean paste and then the eggs, one by one.
Fold in some of the flour, then add some of the buttermilk. Add each in stages and mix in between until all fully added.
Place half of the cake mixture in a deep 8-inch/20 cm springform cake tin sprayed with cake release spray.
Place some pieces of the drained roasted rhubarb on top, then add the rest of the cake mixture on top of it.
Smooth the top then place more of the drained roasted rhubarb on top (you may not want to use it all, I had some left over).
Bake for 50-55 minutes at 180 degrees (preheated oven), or until the cake is golden brown and fully cooked in the centre - check using a cake tester.