Preheat the oven to 180oC/350oF/Gas
4. Line the base of a 23cm square cake tin with greaseproof paper and
grease the paper and sides.
For the topping, place the brown
sugar, cinnamon and walnuts in a bowl. Mix, then add the butter and rub
in until the mixture resembles breadcrumbs.
To make the cake, cream the butter
until soft. Add the sugar and beat until light and fluffy. Add the eggs,
one at a time, beating well after each addition.
In another bowl, sift the flour,
bicarbonate of soda and baking powder together three times. Fold the dry
ingredients into the butter mixture in three batches, alternating with
the soured cream. Fold until blended after each addition.
Pour half of the batter into the
prepared tin and sprinkle over half of the topping. Pour the remaining
batter on top and sprinkle over the remaining topping. Bake until
browned, 60-70 minutes. Leave in the tin for 5 minutes, then turn out
and cool on a wire rack.