Blackberries, lemon and cream are a winning combination, and, if you're up for it, blackberry Napoleon is a relatively easy and fun dessert to assemble using store-bought puff pastry. Once the various components are ready -- the pastry, the lemon curd, the whipped cream and the berries -- it takes only a few minutes to put together. It's basically a dessert sandwich: You mix whipped cream and lemon curd together and spread it over a sheet of pre…