- 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.