Fats don’t soak into meat during cooking although they will cling to the outside. If you aren’t eating the cooking liquid or making gravy, you’re just throwing most of that expensive butter away. Why don’t you try making the roast without the butter next time? You can always take the meat out of the cooking liquid, boil the liquid down a little and add a couple of tablespoons of butter to smooth it out for the people who can afford the high fat sauce.
A long, slow braise melts the collagen in the connective tissue of the meat and gives it a nice juicy texture, no fat needed. I suspect a reduced fat version will be just as tasty.
You definitely should account for the fact that you are cooking the roast in butter. Cooking food in fats (oil, butter, etc.) will add calories to your meal. I can’t tell you that it will be 100% of the butter cooking in, especially since some of the leftover juices will contain some of that butter (and perhaps some of the roast’s fat that has melted off). However, I would recommend using the full amount of butter in your MFP recipe, especially if your goals are related to losing weight or not gaining weight. Even if you do end up underestimating consumption with this method, it will be one meal a month as you have said, so consistency in the rest of your tracking should make this OK.
Eat the butter.
Have you tried the recipe with less butter? Like maybe a stick and half, then if that works, 1 stick?
I make this same roast occasionally. I use 1 packet of au jus, 1 packet of ranch, at least 10 peperoncinis and about 4 tablespoons of cubed butter in a crockpot on low for about 8 hours. Turns out delicious every time. Would probably cut back on the calories a little bit. 🙂