Preheat the oven to 180 ºC (350 ºF). Butter a 20-cm (8-inch) springform pan.
In a bowl, beat the butter, egg, brown sugar and vanilla with an electric mixer until the mixture is smooth.
In another bowl, combine the flour, baking soda and salt. Add to the previous mixture alternately with the buttermilk. Add the rhubarb and combine with a wooden spoon. Pour into the prepared pan.
In a small bowl, combine the sugar and cinnamon. Sprinkle over the top of cake.
Bake in the middle of the oven for about 1 hour or until a toothpick inserted in the centre of the cake comes out clean.
If you don’t have buttermilk, use 250 ml (1 cup) milk with 5 ml (1 teaspoon) white vinegar added. This whitish liquid with a sour taste is in fact partly skimmed milk with added bacterial culture. We usually use buttermilk to give a soft texture to cakes and muffins. Unfortunately, buttermilk is only sold in one litre cartons. Thus, you must keep the remainder, once opened, a week in the refrigerator. For a longer period, freezing is necessary. Use small sealed plastic bags and measure 125 ml (1/2 cups) per bag. This way, if you freeze it flat, it will thaw quickly. I sometimes add some to my pancakes or mashed potatoes preparations. You can also make refreshing drinks with fruit and a little sugar in a blender.