pastry: mix flour and salt,add fats rub in till like crumbs.
add enough water to make stiff dough.
line bottom of 7 to 8 inch shallow sided square tin bake in hot oven for 20 mins,press down halfway through cooking time if need.
filling: melt butter and jam,add cake and lemon with rum in bits,press into tin.
cover and leave in fridge overnight.
icing: next day make stiff paste with icing sugar cover top of slice,leave to dry then cut in slices.
i have found that depending on how dry or moist your cake is you may need more or less butter,it helps to have the cake as stiff as possible,i have found that putting a layer of marzipan on the top before icing is nice.