1
Preheat the oven to 200°C/180°C fan/400F/Gas 6 and grease and line a 33 x 23cm (13 x 9in) baking tray or cake tin.
2
Cream the butter and sugar together until light and fluffy. Gradually add the eggs and vanilla bean paste, mixing well between each addition.
3
Gently fold in one-third of the flour, the baking powder and one-third of the buttermilk or yoghurt; repeat until you have added all of the flour and buttermilk.
4
Bake for 50 minutes until risen and lightly golden and a skewer inserted in the centre comes out clean (this step is very important as ovens can vary). Cool in the tin for 5-10 minutes, then turn out onto
5
a wire rack to cool completely.
6
Meanwhile, make the icing. Beat the butter until light and fluffy then gradually beat in the icing sugar until you have a smooth icing. Add the vanilla bean paste, food colouring and 1-2 tablespoons of just-boiled water. Whisk together to make a bright, smooth icing.
7
Once the cake is cool, transfer to a serving plate or board. Spread the icing all over the cake with a swirly pattern then scatter with the sprinkles and serve.