To melt chocolate using a double boiler, set up a medium saucepan with about a cup of water. Set the water over medium heat and then place a metal or glass bowl over the top of the saucepan. Add the butter and chocolate to the bowl. Once the water underneath starts to heat up, the steam will gently melt the chocolate. Make sure to whisk together the butter and chocolate, as the chocolate is nothing, to ensure it stays smooth. Once the chocolate is fully melted, remove the bowl from the steam and set the chocolate aside to cool slightly before you add the egg yolks.