By Mehnaz Yasmin and Niket Nishant (Reuters) -American Express Co on Friday topped first-quarter profit estimates as card spending rebounded from last year, dwarfing a jump in costs due to higher reward payouts. Expenses rose 34% to $9.06 billion on higher customer engagement costs, the company said, sending shares down nearly 2% in morning trading. Senior executives at AmEx, however, sought to reassure investors that the company’s strategy was paying off. AmEx added 3 million new proprietary cards, with U.S. Consumer Platinum and Gold card and U.S. Business Platinum card customer acquisitions…