Preheat the oven to 350°F. Line the base and sides of a 20cm loose bottomed cake tin with baking parchment and put to one side.
Place the chopped rhubarb in a bowl and top with 2 tbsp of the granulated sugar. Stir and put to one side.
Next make the crumble topping. Place the flour, baking powder and cold butter in a bowl and rub the flour into the butter with your fingertips until you have rough breadcrumbs. Stir in the sugar and put to one side.
Next make the cake. Place the granulated sugar and butter into a large bowl. Using a mixer or a hand whisk, beat the butter and sugar together until pale and creamy. Stir in the eggs, vanilla extract and soured cream. Then gently fold in all but 2 tbsp of the flour, the baking powder and the salt.
Spoon out half of this mixture into the cake tin, and smooth using the back of a spoon. Ensure all of the base is covered.
Sprinkle the 2 tbsp of flour onto the rhubarb, and scatter half of the rhubarb onto the cake mix in the tin. Spoon the rest of the cake mix on top, then scatter on the remaining rhubarb. Sprinkle all of the crumble topping on. Make sure it goes right up to the sides, otherwise the cake mixture will try to rise up the sides of the tin and on top of the crumble topping. Place in the oven for 75-90 minutes – until an inserted skewer comes out clean. It’s worth covering the cake with some tinfoil after about 50-60 minutes to prevent the top from burning.
Once cooked, take out of the oven and leave to cool in the tin for at least 20 minutes. Then remove from the tin and leave to cool further on a cooling rack. Serve with fresh cream.
Notes
This cake will stay fresh for 2-3 days, wrapped and stored in an air tight container at room temperature.