Preheat the oven to 450 degrees . Grease and flour six 6-ounce ramekins. In a double boiler, melt the butter and chocolate over simmering water, whisking until smooth. Remove the insert from the pan.
Using an electric mixer, beat the whole eggs, egg yolks and granulated sugar at high speed until pale and very thick, about 8 minutes. Whisk in the chocolate mixture, then the flour until just combined.
Divide the batter among the prepared ramekins and submerge a stack of 4 chocolate-mint pieces in the center of each. Place the ramekins on a baking sheet and bake until set but still slightly jiggly in the center, 12 to 14 minutes.
Invert the cakes onto plates, wait 20 seconds and remove the ramekins. Dust with the confectioners' sugar and serve immediately.
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