- 225 g (8oz) butter, softened
- 350 g (12oz) caster sugar
- 2 tsp vanilla extract
- 4 eggs, beaten
- 350 ml (12fl oz) sour cream
- 525 g (1lb 2.5 oz) plain flour
- 1 tsp baking powder
- 1/2 tsp bicarbonate of soda
- 1 tsp salt
For the crumb topping
- 150 g (5oz) butter, melted
- 300 g (11oz) plain flour
- 1 tblsp ground cinnamon
- 225 g (8oz) soft dark brown sugar
- 1/4 tsp salt
- 25 x 35cm (10 x 14in) roasting tin
- Preheat the oven to 180°C (350°F/Gas 4), then line the base and sides of the roasting tin with baking parchment, with the paper coming just above the sides of the tin to enable the cake to be lifted out easily.
- First make the crumb. Pour the melted butter into a large bowl or into an electric food mixer. Sift in the flour and cinnamon and add the remaining ingredients, then mix briefly – for just 30–60 seconds if using a food mixer – until coarse and crumbly.
- To make the cake batter, first cream the butter until soft in a separate large bowl or in the food mixer.
- Pour in the sugar and beat until the mixture is light and fluffy. Add the vanilla extract and gradually mix in the eggs.
- Add the sour cream, then (with the mixer on a low speed, if you’re using it) sift in the remaining ingredients, and fold in to combine. Tip the batter into the prepared tin and smooth the surface with a spatula or the back of a spoon.
- Sprinkle over the crumb mixture, making sure it covers the batter completely. Bake on the lower shelf in the oven for 45–55 minutes or until golden brown on top and a skewer inserted into the centre of the cake comes out clean.
- Check the cake after 30 minutes and if the top is already a rich golden brown, cover the tin with foil (to prevent the cake from burning) and continue to cook for the remaining time.
- Remove the cake from the oven and allow to cool in the tin completely. Then lift the cake out using the baking parchment, peel away the paper and cut into squares to serve.