1. Heat oven to 450ºF. Butter two 6-oz ramekins and dust the inside with flour.
2. Place the butter and chocolate in a glass bowl and microwave on high in 20-second intervals, stirring after each, until melted and smooth.
3. Using an electric mixer, beat the egg, egg yolk and sugar in a medium bowl until thick and lighter in color, about 1 minute. Add the melted chocolate mixture and flour and beat until fully incorporated and smooth.
4. Divide the batter between the prepared ramekins. Bake until edges are set and center still jiggles slightly, 8 to 10 minutes. Let stand for 15 seconds. Run a knife around edge, then invert each cake onto a plate. Dust with confectioners’ sugar and serve with fresh mint or berries, if desired. Serve immediately.