This recipe for cannabutter uses The Magical Butter Machine - you can get $25 off yours by using this link here. I highly recommend this machine if you plan on making cannabutter, it's AWESOME.
The first step to making cannabutter is to decarboxylate your marijuana. DO NOT SKIP THIS STEP (to learn why, click here). Preheat your oven to 240. Measure out one ounce or more of "trim" (the leftover product after you grow and trim your marijuana flowers) or 1/2 ounce of marijuana flower. Spread it over a baking sheet so there are no empty spots. Bake for 40 minutes, giving it a gentle stir every 10 minutes. Once it's done, let it cool down and then crumble it up a little finer.
Melt your unsalted butter (one pound, I use organic) on the stovetop in a small saucepan and add it to the Magical Butter Machine. Add 2 Tablespoons of lecithin (I use organic sunflower lecithin) and the decarbed marijuana. Set the temperature for 160 and the timer for 2 hours. Wait for the machine to start beeping at you when it's done - strain your butter into a glass bowl or container and cover. You lose about 1/4 cup of butter in the process, so you'll end up with about 3/4 pound of cannabutter.
If you do not have a Magical Butter Machine (why not???), you can find a recipe with stove-top instructions here.