Heat oven to 160C/140C fan/gas 3 and grease and line the base and sides of a deep 20cm round cake tin with a little of the butter and baking parchment. Put the butter, 250g sugar and vanilla into a big mixing bowl. Beat until light and fluffy with an electric whisk.
Beat in the eggs, one by one, then fold in the 300g flour and baking powder. Spoon out 85g of the batter, and stir the extra 7 tbsp flour and cinnamon into this with a cutlery knife so it becomes crumbly.
Fold the rhubarb into the rest of the cake batter and scrape into the prepared tin. Scatter over the crumble mix followed by 1 tbsp sugar. Bake for 1 hr 15 mins, until a skewer poked in comes out clean – you’ll need to lay a sheet of foil on top after an hour if the cake is browning too much. Cool for 15 mins in the tin, then finish on a wire rack.