Melted butter, to grease
2 Tbsp unsalted butter, softened
1 bunch (530g) rhubarb,trimmed
21⁄2 cups caster sugar
2 cups plain flour
1⁄2 tsp fine salt
1 tsp ground cinnamon
Extra 275g unsalted butter, softened
2 tsp baking powder
Finely grated zest and juice of 1 orange
1 cup sour cream
Crème fraîche, to serve
Preheat oven to 180°C. Grease a 22cm round cake tin with melted butter and line with baking paper. Dot base with softened butter in small pieces.
Cut rhubarb into 5cm lengths. Transfer to a large bowl and toss with 3⁄4 cup of the caster sugar. Set aside for 10 minutes.
Meanwhile, put 1⁄2 cup of the flour and 1⁄4 cup of the caster sugar into a large bowl. Stir in salt and cinnamon, then add 75g of the extra butter. Using your fingertips, rub in butter to form crumbs. Set aside.
Stir rhubarb again, then arrange in base of prepared tin in a decorative pattern. Sprinkle over excess sugar from bowl.
Put remaining butter with 1 cup of the caster sugar in the bowl of an electric mixer and beat, using paddle attachment, until light and creamy. Add eggs, 1 at a time, beating well after each addition. Sift in remaining flour and baking powder. Add zest and sour cream. Stir until combined. Spoon over rhubarb and smooth surface. Scatter crumble mixture on top. Bake for 50 minutes or until cooked when tested with a skewer. Stand for 5 minutes to cool slightly, then turn out onto a serving plate.
Meanwhile, combine juice and remaining sugar in a small saucepan over a medium heat. Bring to the boil. Cook for 5 minutes or until syrupy. Brush over warm cake and serve with crème fraîche.