Preheat your oven to 190C/170C and line a 9" square tin with parchment paper.
Beat together your butter and sugar until light and fluffy.
Add in the flour, eggs and vanilla and beat again until smooth.
Mix your chopped up rhubarb (about inch size pieces) with the other 50g of sugar, and then pour half into the cake mixture. Mix together and pour into the tin!
Put the rest of the rhubarb on top of the cake evenly, and then cover with your crumble mixture! You want it to be no more than 1cm thick otherwise it wont bake properly!
Bake the cake in the oven for 30-40 minutes. It shouldn't wobble when done, and the crumble should be a lovely colour on top!
Leave to cool fully, cut up into squares, and then serve how you like!
Notes
This cake should really be stored in the fridge due to the fruit, but should be fine out on a colder day.
This cake will last 3 days after baking!
My crumble mixture is these ingredients, rubbed together to resemble bread crumbs.
120g plain flour
60g caster sugar
60g unsalted butter
I won't always use the crumble mixture in the cake, but its worth making the ratios the same if you're going to make a smaller amount!
You can use regular rhubarb, or forced rhubarb, but forced rhubarb is generally sweeter and pinker.
I served mine with cream, but you can easily serve it with custard, or evaporated milk for example!