Wes,
God's goodness is rooted in His character (essentialism). God says something is good or evil because it is either consistent or inconsistent with His perfectly good nature.
When we say “God is good” we are not saying God is identical to goodness, but that goodness is a quality of God’s character. That doesn’t mean we are “judging God by a standard independent of God and declaring by that standard that God is good.” Indeed, God’s character sets the standard. It is internal, not external.
But how do we know that standard is a good standard? Greg Koukl comments, “Regardless of how one grounds the concept of goodness, another could always ask, 'But what makes that notion good?' To avoid a vicious regress, one must eventually appeal to some irreducible, primitive concept known by intuition.” So where do we stop? Koukl argues that we know God is good through our own moral intuitions about goodness. When we look at the moral qualities predicated of God, our moral intuition beholds those things and recognizes that they are good. Based on the nature of intuition no deeper level of justification need be, nor can be provided.
It should be noted that essentialism does not exclude voluntarism (the first horn of Euthyphro’s dilemma). When we look at the spectrum of laws God has issued, clearly not all of them have to do with morality. Some commands are amoral in nature (e.g. the command to build a fence on one’s roof), and we are required to obey them on the basis that God said we should. The Israelites would have been wrong to disobey God’s command to build a fence on their roof, even though the command was amoral in nature. The wrongness of their act would not be due to the immoral nature of failing to build a fence on one’s roof, but from a rejection of God’s authority to command His creation.
Both essentialism and voluntarism are necessary to explain the spectrum of commands we find in Scripture. But since amoral commands have nothing to do with goodness, voluntarism (the first horn) is irrelevant to the question of what makes something good. Essentialism explains that.