Make the custard by mixing the custard powder, sugar and milk in a small saucepan.

Cook, stirring, until the mixture boils and thickens.

Remove from the heat and stir in the butter and vanilla extract.

Press cling wrap over the surface of the custard to prevent a skin forming and allow to cool.

Preheat the oven to 180deg C/350deg F gas mark 4.

Grease a 20cm / 8 inch spring-form cake tin and line the base with baking paper.

Cream the butter and sugar until light and fluffy.

Beat in the eggs one at a time adding a little of the flour mixture to stop it curdling.

Fold in the remaining flour and custard powder.

Spread half the mixture into the cake tin and top with the whisked custard.

Carefully spread the remaining cake mixture to cover the custard.

Top the cake mixture with the rhubarb and brush with the melted butter and sprinkle with the extra caster sugar.

Bake in the centre of the oven for around 11/4 hours then allow to cool in the tin.

Dust with icing sugar before serving.