For chewier cookies and bars, we melt our butter instead of following the usual procedure of creaming softened butter with sugar. This simple switch frees up the water content of the butter so that it can freely interact with the flour in the dough or batter. This allows gluten—the protein that gives baked goods their chew—to develop.
While that free interaction is a good thing, too much water can compromise gluten development, so we also like to brown the melted butter, since this not only allows a little of the water to evaporate but also gives the butter a deeper flavor. We also choose unsalted butter, since it typically contains less water than salted butter.