First, make the custard. Tip the caster sugar, cornflour and vanilla into a mixing bowl and add the eggs and egg yolk. Whisk with a hand whisk until smooth and blended.
Pour the milk and double cream into a pan and heat gently until hot, but do not boil. Pour this over the egg mixture and immediately whisk until blended. Wash the pan to remove any scum from the milk and pour the custard back into the clean pan. Heat gently, stirring constantly until thickened.
Don’t panic if the custard starts to look lumpy as it thickens, just continue to heat and whisk until the custard is thick and smooth. Pour into a jug and cover the surface of the custard with cling film to prevent a skin from forming. Set aside to cool.
Tip the stoned cherries into a pan with half the jam or conserve and cook over a medium heat for 5-10 mins until softened and saucy. Remove from the heat and leave to cool slightly.
Spread half the slices of Madeira cake generously with the remaining cherry jam. Sandwich with the remaining cake slices, then cut in half.
Pour the kirsch or brandy into a shallow bowl, dip each sponge sandwich into the liqueur, then arrange in the base of the trifle dish (you will need a shallow, straight-sided, 20cm round glass dish about 5cm deep).
Line the edges of the dish first to create a stripy effect, then fill in the centre with the remaining sandwiches, pouring over any remaining liquid.
Spoon over the cherries and any juice from the pan, then scatter over the broken biscuits. Pour over the cooled custard, then spread whipped cream over the top. Decorate with fresh whole cherries and chill for a few hours, or make it a day ahead and chill for up to 24 hours before serving.
Decorate with fresh whole cherries and chill for a few hours, or make it a day ahead and chill for up to 24 hours before serving.