Preheating the oven
Buttering the ramekins
Preheat the oven to 350°F, positioning a rack in the middle of the oven.
In a microwave-proof bowl in the microwave oven, or in a small saucepan over low heat, melt 3 ounces of the butter. With a brush, generously butter the insides of 8 (4-ounce) ramekins or other round ovenproof baking containers. Arrange on a baking tray. Set aside.
Melting the chocolate
In a bowl, combine the chocolate and remaining butter. Melt over a double boiler.
Mixing the batter
In the bowl of a stand mixer, using the wire whisk attachment, beat together the eggs, egg yolks, and sugar for 3 minutes over medium speed. Add the warm chocolate and butter mixture, scraping it into the bowl with a rubber spatula. Mix at low speed until well blended. Stir in the flour.
Baking the cakes
Spoon the batter evenly into the 8 buttered cups. Bake for 5 minutes. Rotate the baking tray 180 degrees and continue to bake 3 minutes longer, until the sides of the cakes look firm but the centers still seem moist and look slightly sunken.
Resting the lava cakes
Remove the tray from the oven and set aside for 5 to 10 minutes.
Serving the lava cakes
To serve, use a flexible oven glove or heatproof pad or towel to grip a cup firmly and invert it onto a serving plate; lift off the cup to unmold. (If it won't unmold, use a thin knife to loosen its edge all around the side.) Repeat with the remaining cakes. Serve immediately, with whipped cream or vanilla ice cream.