Rhubarb Crumble Cake

I was making rhubarb crumble the other day, the first of the year now that my rhubarb is doing so well. As often happens I made more crumble than I needed so I put 100g of it in a container in the fridge having stirred in a heaped teaspoon of ground cinnamon. I planned then to make some sort of streusel topped cake. This is the cake.

Nigeria makes it 97, welcome.

For this recipe you will need,
230g of unsalted butetr at room temperature
230g of light muscovado sugar
230g of self raising flour
4 small eggs (my Barnevelder chicken lays the perfect size)
1 teaspoon of good vanilla essence
1 pinch of salt
100g of rhubarb, finely sliced
1 tablespoon of caster sugar
100g of crumble mixture made from:
50g of plain flour
40g of unsalted butter
10g of caster sugar 
1 heaped teaspoon of ground cinnamon

Begin by slicing up the rhubarb and dredging it in the tablespoon of caster sugar, set aside while you make the cake batter. Prepare a loose bottomed cake tin measuring 21cm by 7cm by lining it with baking parchment. Heat the oven to 180C. Cream the butter and the sugar and add the eggs one at a time, beating between each addition. Add the vanilla and fold in the sifted flour and finally add the sugared rhubarb. Place the mix in the cake tin and sprinkle on the crumble topping. Bake in the oven for 40 minutes, switch the oven off and leave the cake in the oven for a further 20 minutes. You should find the cake will rise up and capture some of the topping, I'm sure there are recipes out there that help you avoid this but personally I like the marbled effect you get when this happens.

Notes:
This cake as you will imagine works perfectly well with the use of apple (diced) instead of rhubarb, If you use apple you can omit the sugar dredging stage.