Preheat oven to 350 degrees. Lightly grease an 8-by-8-inch baking pan with butter and set aside.
Set a metal bowl over a pot to create a double boiler. Add enough water to pot to reach just below bottom of bowl. Bring water in pot to a steady simmer over medium heat. Melt chocolate, butter and miso together. In a medium bowl, whisk together sugar, flour and cocoa power. Gradually whisk dry ingredients into melted chocolate mixture. Remove batter from heat and whisk in eggs one at a time.
Pour batter into pan and bake until a cake tester comes out clean, 35-40 minutes.