Preheat oven to 425 degrees. Combine kale leaves, olive oil, salt, and garlic powder in a large bowl and toss well to coat all the leaves. Spread in a single layer on baking sheets lined with Silpat, foil or parchment paper. You will probably need two large baking sheets. Bake for 7 minutes. Remove from oven and remove any crispy pieces. Toss the remaining kale leaves on the baking sheet and spread in single layer. Continue baking another 5 minutes. Once again, remove from oven and remove crispy pieces. If any pieces need to be crisped up more, return them to the oven for another few minutes. Sprinkle with sherry vinegar just before serving, if desired (kale will not be ask crisp if you do this).