Preheat oven to 190 C / Gas mark 5. Butter a 23cm (9 in) round cake tin or springform tin.
Using a fork, work flour with brown sugar and butter in a large bowl until crumbly. Stir in nuts. Divide mixture in half. Evenly press half into tin bottom to form a base.
Stir bicarbonate of soda, cinnamon and salt into remaining crumb mixture until blended, then make a well in the centre. In a small bowl, lightly beat egg with soured cream and vanilla until smooth. Add to flour mixture, stirring just until combined. Fold in apples.
Evenly spread batter over crumb mixture. Bake until a cake tester inserted into centre comes out fairly clean, about 1 hour and 20 minutes. If top gets too brown before cake is baked, lightly lay a piece of foil over top for last 10 to 20 minutes of baking. Cool in tin on a rack. Serve with Brown Sugar Sauce. Cake keeps well at room temperature for a day. Covered, it freezes well and is easily cut while still frozen.
Great recipe. Really easy and very moist. Didn't need the full time though. The sauce was straightforward and creamy - I added a bit more milk for the consistency to pour. Very sweet though. I might try it again with a little less sugar. - 23 Sep 2013