Preheat the oven to 180°C (Gas mark 4) and prepare an 8-inch round tin by greasing and lining it.
For the crumble topping, sift the flour into a bowl, then cut the butter into cubes and rub it into the flour until the mixture resembles breadcrumbs. Stir in the brown sugar and set aside. You can also use a food processor for this step.
In a large bowl, combine all the ingredients except for the rhubarb and mix until the batter is smooth and creamy. An electric mixer works best, but you can also use a wooden spoon to mix by hand.
Chop the rhubarb into small pieces and fold it into the batter. If desired, you can add a teaspoon of cinnamon at this stage.
Pour the batter into the prepared tin, using a spatula to scrape the sides of the bowl to ensure all the mixture is transferred. Smooth the top gently with the back of the spatula.
Sprinkle the crumble mixture evenly over the top of the cake batter.
Bake in the center of the preheated oven for about 40-45 minutes, or until the top is golden brown and a knife or skewer inserted into the center comes out clean. If the cake browns too quickly on the outside while still being undercooked inside, cover it with a sheet of tinfoil and return it to the oven.
Once baked, let the cake cool in the tin for a few minutes before removing the baking parchment.
