First prepare the 'filling'. Blitz the nuts, sugar and cinnamon together (or chop the nuts and mix with them). Set aside.
Grease a 8 or 9 inch deep cake tin.
Preheat oven to 350oF or 170oC (slightly less for a fan oven)
Cream the butter and sugar together until smooth, add the eggs, vanilla and beat, then fold in sour cream until well mixed.
In another bowl, sieve flour, soda, baking powder, salt together, make a well then add the wet ingredients. Fold carefully together with a fork until just mixed. Don't over mix or beat, it will spoil the texture.
Spoon 1/3 of the mix into the cake tin, spread out, sprinkle 1/3 of the nut mix over it. Spoon another third over and sprinkle another 1/3 of the nut mix. Spoon the rest on top and sprinkle the last of the nut mix on the top.
Place in over for about 45-50 mins. Don't be tempted to look at it by opening the over before 45 mins, or the cake may sink and you'll be very sad. Have some faith and leave it alone.
Check cake by sticking a skewer or toothpick into it, if it comes out dry it is ready. If it is sticky, another 5 mins and repeat.
Open over door and leave cake on shelf with door open for 5 mins, then move to the top of the oven to cool properly. Leaving it in the warmth of the oven for a while prevents it from shrinking dramatically in the first 5 mins :)