Grease a 9" cake pan. Line the bottom with a parchment circle and butter the paper. Dust with flour and tap out the excess. Preheat oven to 350 degrees.
Place chocolate in a heat-safe bowl. Place the bowl over a pot of simmering water and heat gently until the chocolate melts. Remove and stir to smooth mixture. Let cool slightly.
Place butter and sugar in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed for 4 minutes, scraping down the sides of the bowl frequently, until the mixture is creamy. Add the eggs one at a time and beat well after each addition.
Reduce mixer speed to low and add the cooled chocolate. Mix until incorporated.
With mixer still on low, add the flour and mix briefly, or fold in with a spatula.
Pour batter into pan and smooth the top. Bake 27-35 minutes. A cake tester or knife inserted into the middle will come out mostly clean, and the cake will begin to pull away from the sides of the pan. Remove from the oven and let cool, then refrigerate for an hour or so.
Run a knife around the edge of the pan, place a rack on top and invert the cake onto the rack. Remove the parchment, and invert again, transferring the cake to a serving platter. Dust with confectioners' sugar if desired, and/or berries and whipped cream.
