They may be pennies a packet, but the ubiquitous custard cream can be a revelation if you make them at home. But who has come up with the ideal bake?Prue Leith reckons the custard cream is “arguably Britain’s most iconic biscuit” – and, certainly, we’ve been dunking this fern-patterned treat in our tea for well over a century, with early advertisements for this “delicious biscuit” placing it, perh