5 year old daughter is MEAN and I'm officially at a loss

My daughter has always "felt emotions very strongly" (as described by her daycare teachers). As a toddler she would tantrum so long and so hard she would vomit routinely. She gets mad over perceived slights, or for seemingly no reason at all (the other month she gave me the silent treatment for 2 hours after I dared to nod my head to the music in her dance class). We've read ALL the books, have tried to validate her feelings, and tried different parenting techniques (stern vs empathetic vs consequence, ect). We've tried 2 different play therapists and 1 therapist who specializes in OCD and anxiety. We've been told she's perfectly normal, albeit sensitive and smart.​She can be the SWEETEST kid in the world when she's in a good mood. Adorable, precocious, helpful. But now that she's getting older the tantrums are being replaced by just general meanness, or sometimes she's incredibly mean and hurtful just to test the waters. For example, we were having a perfectly pleasant family dinner when she turned to my husband and asked "Daddy, why did you want to marry mommy? She's so fat". Or another day when she asked "How do people fall in love?" I explained that they meet, find each other attractive, have mutual interests, and like spending time together. She asked me 'Well, you and daddy aren't attractive so how did YOU fall in love?"​I could excuse those as somewhat funny jabs or (seemingly) innocent remarks, but she's also incredibly spiteful when upset. Refuse to let her watch another TV show? Prepare to hear "I don't love you. I'm going to move away from you and you'll be all alone and I'll finally be happy". Don't let her have a brownie before dinner? Get a picture drawn of the family (minus me), with the explanation that she wishes I wasn't her mommy and she wishes she could have any other mommy but me.​We don't talk like this at home. My husband and I are not intentionally cruel to each other. I try to explain each and every time how much that hurts my feelings and how she'll always be my daughter, even when she's mad or old. We talk about how words like that push people away. I've tried being incredibly stern when she tries to throw daggers. I'm trying to let this roll off my back (and it's relatively easy given how hot and cold she is), but I'm truly afraid that as she progresses through elementary school it's going to be a huge issue, especially with making and retaining friends. How do I get a handle on this? via /r/Parenting http://bit.ly/31VTOhy