Preheat the oven to 400 degrees. Generously butter six 4-ounce molds or ramekins.

In the top of a double boiler over barely simmering water, melt the butter and the chocolate. Stir to combine and remove from the heat. Alternatively, put the chocolate and butter in a glass measuring cup or similar container and microwave on medium-high power for 50 to 60 seconds. When the butter melts and the chocolate softens so that it looks shiny, stir until smooth. Microwave for another 30 or 40 seconds, if necessary.

In the bowl of an electric mixer fitted with the whisk attachment, beat the eggs and sugar together on high speed until the mixture increases in volume and is pale yellow in color, about 5 minutes. Reduce the speed of the mixer to low and add the flour. Pour in the melted chocolate and butter mixture and mix well.

Fill the buttered molds halfway with the batter. Put a ganache ball in the center of each mold and then fill them with batter so that they are three-quarters full and the ganache is covered. Refrigerate for at least 30 minutes or up to 2 to 3 days or freeze until ready to use.

Bake the cake for 12 minutes or until the tops are set, look dry, and your finger leaves a mark in the top of a cake mold when gently pressed. (If cakes have been frozen, bake for 14 minutes.)

Allow the cakes to rest in their molds for 1 minute. Run a small, dull knife around the edge of each mold and unmold onto serving plates. Serve with whipped cream or ice cream, if desired.