I can't believe someone is advocating 'whooping the butt' of a two year old. All it teaches is not to quell the behavior, but to fear a parent or adult caregiver.
This is not an 'epic parenting failure' on your part...it is a mistake a lot of parents make-- what to do when a strong-willed child is angry and having a tantrum. Hitting children for expressing their anger is really about the adult being out of control and not knowing how to manage their own feelings. It is not any sort of research-backed parenting method.
Please, find some parent education materials for yourself and your husband. I worked with a group of toddlers for a couple of years-- this behavior is incredibly common. It's not about being a spoiled brat, it's about the toddler's still-developing brain and extremely limited ability to rationally process what's happening.
In short, this is about brain development as well as temperament. "The Science of Parenting" (all about neuro development in young children) would tell you that when her desired item is taken from her, she is angry, which actually triggers the 'pain' part of what we would call her "lower/primitive" brain. Hence, her flight or fight reaction-- she chooses to fight.
When she is acting out, put her in a safe place (pack-n-play or crib) and walk away. Do not give her what she wants. Tantrums cannot be reasoned with or 'disciplined' out of a toddler. In my toddler room, we had multiple items (baby dolls, trucks, toys) so that two or more children could play with them at a time. Don't go back to her until she's done acting out. You may want to offer her something else to do, a distraction (not the item fought over) so that, when she's calm, she can start to relax and keep herself busy. You are teaching her that substitutes are okay and that she doesn't get what she wants when she screams.
Before it escalates: You can try substitution. "Jo-Jo is playing with that one. See? His hands are on it. You may play with this one." this is the adult managing the situation. Another idea that we caregivers lean on is "adjust the environment". This would mean, in your case, that your toddler and infant should not be left alone together or be unattended. This could be ensured by using a pack-n-play for one child, a high chair, other containment options (I used an umbrella stroller when my son was getting into mischief while I was cooking dinner.... buckled him in, gave him a couple toys)... it's not a punishment, it's about managing the space to eliminate conflict.
She's going to need a lot of consistent reinforcement and redirection. Better to stop the judgment on dad's part. It's very typical behavior.... I wonder if your husband would feel this way about a boy exhibiting the same behavior. She's just doing what toddlers do, it's up to us to help them learn about their world in safe, appropriate ways.