Last month I (re-)invented a new thing. A cake sandwich. You know, those packet sandwiches you get in M&S and Pret, but instead of dry bread and uninspiring fillings, I sprinkled some cake magic on the whole thing. The result – the best cake sandwich in the world! Who invented cake sandwiches? Cake sandwiches have been made before - Queen Victoria was the OG with her eponymous Victoria Sponge cake. There have been novelty versions and basic buttercream 'n' jam versions too. But the reason my…