Preheat oven to 200 C / Gas 6. Lightly grease one 26cm tube cake tin.
Cream 300g caster sugar together with eggs until well blended. Add soured cream and butter and beat well. Add flour, bicarbonate of soda and baking powder; mix well. Stir in vanilla and the chopped nuts.
Mix the remaining 4 tablespoons caster sugar with the cinnamon.
Pour half of the batter into the prepared tin. Sprinkle generously with the cinnamon sugar mixture. Cover with remaining cake batter.
Bake in preheated oven for 6 for 8 minutes. Lower heat to 180 C / Gas 4 and bake for an additional 40 minutes.