Well, first get yourself a brownie mix/recipe that takes butter.  Aim for about 1/4 cup of butter.

Next, bring a pot of water to a boil.  Chop up your plant material, and throw it into the pot along with the butter.  Cover, and let it boil at medium heat for about 45 minutes.

Next, get a metal strainer and use it to separate out the boiled plant material.  Squeeze as much liquid as you can out of the plants.  Put the liquid in the fridge for a few hours, until the butter floats to the top & solidifies.  Lift it off, and you've got Budder!  Now use it in your brownie recipe.

A warning: making budder this way makes your entire kitchen stink of weed.  Expect it to take at LEAST a few hours for the smell to dissipate.