Ok. Let's break it apart. At 100c, the water is not "gone", that is the temperature of the water we're talking about. What is the temperature of the pot the water is boiling in?
What the is the temperature of the skillet we are basting in? Probably well above 100 c. However if you add butter to a hot skillet it does not immediately vaporize the entire content of water. In much the same way that once water reaches the "boiling point" it is not immediately all converted to steam. The rate at which this happens is basically unknown any time you're talking about cooking in a casual environment as you must calculate the rate at which heat is given to the water by the heat source (basically getting into fairly advanced thermodynamic calculations at this point). Suffice to say the science of what goes on in a skillet and in particular with a technique like basting a steak with butter is significantly more complex than the picture you're trying to paint.
I will also reiterate that I am intentionally painting a simplified picture, and not taking into account that boiling point can be affected by suspending the water in an emulsion, having sufficiently impure water, etc. etc.
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