Any behavior that gets attention will continue. Imagine a garden: your child is the rose that needs just the right amount of sunlight and water; the dandelions are the unhelpful behaviors, such as tantrums. If you so much as blink in a dandelion’s direction, you know that you will have a garden full of dandelions. This is why after validating once, the next step is to ignore.
Some parents are concerned that they aren’t doing anything when they ignore. You are; you are ignoring actively, which takes effort. This will be very tough. Expect the behavior to get worse before it gets better (what is known as an “extinction burst”). Remind yourself that you are ignoring the dandelions and not your child. Pay attention to anything else: pick the lint off your sweater, do the dishes, or count the clouds in the sky. Do not water the dandelions, though. If you ignore actively for 10 minutes and then eventually shout at your child or just give the child the cookie, the child will learn that he needs to push longer to get attention or the desired outcome. Then you will have even more dandelions in your garden.