Compound butter is pretty easy to make, though it's not something that you see served at home a lot. There are restaurants that throw a little garlic and chive into whipped butter and customers will practically eat it like ice cream. Whatever you decide to blend in, just make sure that it has some time to rest so that the flavor permeates the butter.
For this experiment, I made red wine compound butter. Often finely diced shallots are added, but I just kept it simple with red wine and one stick of unsalted butter.
Take 125 mL or half a cup of red wine and boil it down into a concentrated syrup. Do this slowly and pay attention, because right at the end there's a very fine line between syrup and having to scrape carbon off the bottom of your pan.
You don't want your butter too hard or too soft, just keep mashing it with a sturdy spoon or paddles until you're able to work with it. While waiting for the wine syrup to cool off, feel free to experiment with some small scale butter sculpture.
Put on a shirt that you don't like and begin to incorporate the syrup into the butter. Why does the shirt matter? The syrup and butter don't really want to mix, and you'll end up with little squirts of highly concentrated wine going everywhere. I just used a butter knife for everything, working on top of a sheet of parchment paper. I had a paper towel nearby to grab stray drops before they stained the table. Keep working at it, and eventually you'll get a nice mauve pile of butter that looks like black cherry ice cream. Don't be afraid to grab a small slice of baguette to perform some quality control testing.
Using plastic wrap or parchment paper or wax paper, form the butter into a log. It doesn't have to be perfect, but then you roll it up in the wrapper, and twist the ends to make sure it's tight. With just a little rolling on the table, you can get a solid, nearly perfect cylinder. Throw it in the refrigerator and let it rest for at least six hours before serving.
Any compound butter involving fresh herbs will really rely on this time to develop, but won't last as long. I've made sage butter in the past, and it's amazing on top of a grilled pork chop.
Here we see a finished disc of red wine compound butter on top of a rare ribeye steak, accompanied by a bit of broccolini and the drooling admiration of Wolfgang.
When you rub the compound butter over the top of the steak, it quickly melts and then the magic happens. Some of it sinks down into the meat, but the rest of it picks up the melted fat and salt and other seasonings on the steak. As you're eating, it all runs down the sides and combines with the released juices of the steak to form a sauce on the plate.
What wine to use? In a perfect world you use the same wine that you'll be serving with the steak, but frankly this is a better opportunity to use up some leftover wine that has lost its sparkle, or if you freeze leftover wine for cooking purposes, use some of that. I used a Syrah for this one, but really anything will work. By the time you've boiled it down you've killed off any nuance or subtle character and are left with highly concentrated grape juice.