Place the butter in a small, heavy-bottomed saucepan set over the lowest possible heat. Add 6 ounces (170 g / 1 cup) of the chopped chocolate and melt together, stirring frequently to prevent the chocolate from scorching. Continue cooking until the mixture is pleasantly warm, but not super hot, to the touch. Remove from the heat and keep warm. Whisk together the buckwheat flour, tapioca flour and baking powder in a small bowl and set aside.