Grease the bottom of an 8-inch cake pan and line with parchment paper.
Preheat oven to 350-degrees.
Break up chocolate and put in the top of a double boiler or in a bowl over simmering water. Make sure the water does not touch the bottom of the bowl. Stir to melt. Remove from heat and let cool to slightly warm.
In an electric mixer with a paddle attachment, cream butter and sugar until the mixture is pale, light and fluffy. Slowly add the egg yolks and continue beating to a light consistency.
Add cooled chocolate and beat until smooth and velvety.
Beat egg whites with a completely clean wire whisk attachment until soft peaks. Gently fold egg whites into chocolate mixture.
The egg whites should be gently folded in with a spatula. Make sure there are no white streaks in the batter.
Pour the batter into the prepared pan. Bake for 30-35 minutes or until a toothpick comes out clean. The top will be puffy and may crack.
Remove from oven and cool completely on a wire rack.
Serve at room temperature with whipped cream and a few fresh berries. Alternatively, dust the top with powdered sugar.