Preheat the oven to 350°F. Butter a 9 ½ inch deep springform pan or a deep cake pan.
Combine the chocolate, butter, and sugar in the top of a double boiler. Cook over barely simmering water until everything has melted and is smooth and well combined. Set the mixture aside to cool.
When the chocolate has cooled to room temperature, whisk in the egg yolks one at a time. Whisk in the flour.
Place the egg whites in a clean large bowl and beat just until the form stiff peaks; do not overbeat.
Add one third of the egg whites to the chocolate mixture and whisk until combined. Change over to a silicone or rubber spatula, then add another third of the beaten egg whites, and gently fold them in until just incorporated. Repeat with the remaining egg whites.
Scrape the batter into the prepared pan and bake until the cake is firm and springy 35 to 40 minutes.
Cool in the pan for several hours until it reaches room temperature. Unmold from the springform pan, or carefully turn it out of the cake pan. It will puff up a bit when it bakes, and then settle a bit in the middle, which is just fine.
Dust the cake with confectioners’ sugar. Top with whipped cream and strawberries as desired.
Notes
Serve simply dusted with powdered sugar or go all out with mounds of whipped cream.