When melting chocolate and butter, you want to start by melting the chocolate alone in a double-boiler, then add the butter and finish mixing. The double-boiler keeps the chocolate from coming into direct contact with the heat source, reducing the chance of scorching or burning. Add the butter after the chocolate has begun to melt, as this will keep the butter from separating and allow the chocolate and butter to mix thoroughly.
Fill a large pot 1/3 of the way with water and bring it to a boil over medium-high heat.
Fill a second large pot with small pieces of chocolate and place it on top of the first pot. They should fit together in a way where the top pot rests in place about 1/2 inch above the boiling water.
Stir the chocolate constantly. As soon as the chocolate begins to melt, remove the pan from the heat source.
Add the butter to the chocolate, stirring to mix the two thoroughly. Work quickly to allow the chocolate to melt the butter before it cools down.