For the cake
Butter or oil spray to grease cake tin
400g rhubarb, trimmed and cut into ½ cm-thick slices on the angle
1 cup sugar, divided
1 cup white flour
1 tsp baking powder
75g butter, softened
1 tsp finely grated orange zest
2 large eggs
½ tsp vanilla extract
¼ cup sour cream
For the crumb topping
½ cup plain white flour
¼ cup soft brown sugar
1 tsp ground ginger
25g butter, melted
Preheat oven to 180degC/160degC fan assisted, with rack one level lower than centre.
Lightly grease 18cm square cake tin with cooking spray or butter and line with greaseproof paper, leaving an overhang on two sides (this acts as a handle).
Stir together rhubarb and half cup of sugar; set aside to macerate.
Whisk together flour and baking powder. Beat together butter, remaining half cup of sugar, and the orange zest until light and fluffy. Beat in eggs, one at a time, then beat in vanilla.
Beat in flour in two additions, alternating with sour cream, beginning and ending with flour.
To make the crumb topping: Stir together flour, ginger, brown sugar, add melted butter; stir to combine.
Pour cake mixture into prepared tin. Top with rhubarb mixture, and sprinkle with crumb topping.
Bake until golden on top and cooked through, about 45 minutes. Check the cake, however, as it can take a little less time or sometimes a little more. Let cool completely in tin on wire rack, then lift cake from pan using greaseproof paper.
Remove greaseproof paper before serving. Cut buckle into 4cm squares.