Don't you like chocolate? Cocoa stains occur when the cocoa in chocolates, which is a source of motivation and happines, contamined to clothes and laundry. Waiting a few hours will cause the cocoa to penetrate into the fabric fibers and cleaning would be difficult. You may leave it for a little while on the stain with liquid detergent. Then wash it in the washing machine with the smart detergent Peros Matik for Whites and Brights.