Start this recipe the day before serving. To make the dough, put the flours and butter in a bowl and use your fingertips to mix them together, until the butter is evenly dispersed and the mixture forms crumbs
Add the sugar, egg yolks and sour cream and mix together using your hands or a wooden spoon. When a soft, sticky ball of dough is formed, wrap in plastic and refrigerate overnight.
To make the crumble, use your fingertips or a food processor to combine the flour, sugar and butter until crumbs form. Refrigerate until needed.
Remove the dough from the fridge 30 minutes before using.
Preheat the oven to 180C (350 F). Grease a 28 – 30 cm springform cake tin.
On a well-floured surface, roll out the dough thickly, to fit the base and halfway up the side of the tin. Line the base and side of the tin with the dough, then place the plums on top, arranging them very close together.
With your hands, squeeze the crumble mixture together, then break it up over the top of the plums. Sprinkle with the cinnamon sugar.
Bake for 11/2 hours, or until the cake is golden on top and cooked through.
Serve warm or at room temperature with cream or ice cream.