Yes. You can do an 1:1 substitution for oil with butter in brownies.

Please Note:

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Depending on the recipe, there might be none to a little difference in the texture of the brownies. The difference might arise due to the fact that Oil is 100% fat whereas butter is about 80% fat. So the brownies made using oil might be richer and more silkier while the ones with butter might be cakier. If that bothers you, try removing some of the additional moisture in the butter by browning it first before adding to your brownie recipe.

This particular tip comes from Mr Vittles @ Food52.com