Preheat the oven to 180°C. Grease and line an 8 inch springform cake tin.
In a large mixing bowl, cream together the butter and sugar until pale and fluffy. Combine the flour, salt and bicarbonate of soda in a mixing bowl and add a third of it to the sugar and butter and mix well. Add a third of the buttermilk, mix well, then add half the remaining flour mixture.
Add half of the remaining buttermilk, mix well and add the remaining flour and mix well again. Add the rest of the buttermilk and mix well. Toss the rhubarb in the reserved tablespoon of flour and fold into the cake batter.
Spoon into the prepared tin and bake for 45-50 minutes until a cake tester comes out clean when inserted into the centre of the cake. Remove from the oven and dust immediately with the reserved caster sugar. Allow to cool in the tin for 5 minutes before turning out onto a wire rack to cool completely.
Place on a lined chopping board and serve.