Equivalent of do() while{} in Parallel

You could force a single execution by 'prepending' a null/default value:

static IEnumerable<T> YieldOneDefault<T>(this IEnumerable<T> values)
{
    yield return default(T);
    foreach(var item in values)
        yield return item;
}

And then use it as follows:

Parallel.ForEach(TestBuffer.YieldOneDefault(), Item =>  
{  
    if(Item != null)
      TestBuffer.RemoveItemAndDoSomethingWithIt();
    else
      DoSomethingDuringTheFirstPass();
});  

Although I suspect you might be looking for the following extension methods:

public static IEnumerable<IEnumerable<T>> GetParrallelConsumingEnumerable<T>(this IProducerConsumerCollection<T> collection)
{
    T item;
    while (collection.TryTake(out item))
    {
        yield return GetParrallelConsumingEnumerableInner(collection, item);
    }
}

private static IEnumerable<T> GetParrallelConsumingEnumerableInner<T>(IProducerConsumerCollection<T> collection, T item)
{
    yield return item;
    while (collection.TryTake(out item))
    {
        yield return item;
    }
}

Which will let get you this result (which I think is what you are after):

Parallel.ForEach(TestBuffer.GetParrallelConsumingEnumerable(), Items =>       
{
    foreach(var item in Items)
    {
       DoSomethingWithItem(item);
    }
});