I've always had the most success using straight butter and ground bud in a crockpot on "warm setting" for 24 hours. Start it on "low" or "high" then let it sit on  the "warm setting. Let cool until you're able to handle it safely, strain it (I use heat resistant gloves to strain it through a Cotton T shirt while it's still hot), put in desired container and refrigerate. You should be left with a nice dark green puck of cannabutter  I have done it with successful results and some of the most potent highs of my life with up to 28 grams of bud per pound of butter.