I'm in no position (knowledge-wise) to offer a useful take on whether there is/isn't an ultimate ground per Buddhism.
I have a Tibetan Buddhist friend (Lama Surya Das student) who tells me that awareness is the ultimate ground.
But I've read a bunch that says emptiness goes all the way down ... i.e. there is no ultimate ground. This is actually what drew me to Buddhism in the first place, that there is no final answer, that the notion itself of a coherent question/answer about ultimate truth is conceptual, therefore essentially a fairy tale.
When there is this, there is that. When there is not this, there is not that.
From the arising of this, that arises. From the ceasing of this, that ceases.