I cannot see which is italicized so I will say which one is cause and which one is effect: The effect is that the cookies in the oven became charcoal. The cause of why the cookies became charcoal is that 'David left the cookies in the oven too long.'
The artist might be trying to convey that the subject of the song has been in love with the singer for a while now but the subject is too shy to admit it. Hope this helps! :)
