Grease and double-line the base and sides of a 22cm round springform cake tin. Preheat oven to 170°C.
For the date filling, remove the seeds from the dates. Combine pitted dates, sugar and 1 1/2 cups water in a saucepan. Bring to boil, then reduce heat and stir constantly for about 5-6 minutes, until mixture is thick. Transfer to a bowl and set aside.
For the crumble, blend flour, cinnamon, butter and sugar in a food processor, using the pulse button until mixture resembles large breadcrumbs. Roughly chop nuts and stir into mixture.
For the cake mixture, sift together the two types of flour, set aside.
Using an electric mixer, beat butter and sugar until light and fluffy, then beat in vanilla extract. Add egg, then egg yolk, beating well after each addition. Stir in one-third of the sour cream, then one-third of the flour mixture and mix well. Repeat process twice more to use all the sour cream and flour mixture.
Spoon the cake mixture into prepared cake tin, smooth out top. Spread date filling over batter, then cover with crumble.
Bake for 70 minutes, or until a skewer inserted into the centre comes out clean. Set aside to cool on a cooling rack.
Cut cake using a hot, clean knife and serve with whipped cream on the side.