Steps
- Preheat the oven to 325°F / 160°C. Grease a deep 8in / 20cm round cake pan and line its base and sides with parchment.
- Beat 250g of softened butter, 250g of golden superfine sugar, and 2 teaspoons of vanilla extract until pale and fluffy. Beat in 5 large eggs one at a time, then fold in 300g of all-purpose flour and 2 teaspoons of baking powder.
- Transfer 85g of batter to a small bowl. Mix in ¼ cup plus 3 tablespoons of all-purpose flour and 1 teaspoon of ground cinnamon with a table knife until crumbly.
- Fold 300g of trimmed finely sliced rhubarb into the remaining batter and spread in the pan. Scatter the crumble over it, followed by 1 tablespoon of golden superfine sugar.
- Bake for about 1 hour 15 minutes, until a skewer inserted in the center comes out clean. Check after 1 hour and loosely cover with foil if browning too much.
- Cool in the pan for 15 minutes, then transfer to a rack to cool completely.