Caught my 6yo lying about eating cookies - how do I handle this?

So my son told me he didn't eat the cookies that were clearly all over his face and hands. I know kids test boundaries but I want to make sure I'm modeling honesty and teaching him right from wrong the way God would want. Do I punish him or use it as a teaching moment? My wife and I pray with the kids every night and I really want to raise them to fear the Lord and tell the truth. What have you guys done in similar situations?