1. Preheat the oven to 170°C (fan)/190°C/375°F/Gas Mark 5. Grease a 20cm square or rectangular cake tin with butter and line with baking paper
2. To make the cake, cream the butter and sugar together until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract
3. Sift the flour and salt together, then fold into the mixture. Stir in the Madeira wine
4. Pour the mixture into the prepared tin, then scatter the sliced rhubarb on top
5. Pour the ready-to-pour custard over the rhubarb in the cake tin
6. To make the crumble topping, rub together the butter, caster sugar, and flour until it resembles coarse breadcrumbs. Sprinkle the crumble over the custard layer
7. Bake for 50-60 minutes or until a skewer inserted into the centre of the cake comes out clean. Allow to cool completely in the tin before removing
8. To make the icing sugar drizzle, sift the icing sugar into a bowl and gradually stir in enough water to form a smooth, runny icing. Drizzle the icing over the cooled cake.
NOTES
Be careful not to overmix the cake batter, as this can lead to a tough texture. Only mix until the ingredients are just combined. Use fresh rhubarb for the best flavour and texture. If using frozen rhubarb, be sure to thaw it and drain any excess liquid before using. The crumble topping can be made ahead of time and stored in the fridge until ready to use. Be sure to allow the cake to cool completely in the tin before removing it, as it can be quite fragile when warm. When pouring the custard layer, ensure that it is spread evenly to avoid any gaps or uneven distribution in the finished cake.
