Make ganache: Place 4 ounces chocolate in a small heatproof bowl. In a small pan over medium-high heat, warm cream until it begins to boil. Pour cream over chocolate. Let stand a few minutes, then mix until smooth. Cover with plastic wrap and refrigerate until set, about 1 hour. Use 1-inch ice cream scoop to make balls; place on parchment-lined tray, close but not touching. Freeze 1 hour, or airtight up to 3 days.
Make batter: Preheat oven to 450 degrees. Line a muffin tin with paper liners. In a medium bowl set over a pan of simmering water, heat remaining 8 ounces chocolate with butter until melted. Meanwhile, with a mixer, whisk together eggs, yolks, and sugar until pale and thick, about 5 minutes. Quickly fold egg mixture into chocolate until almost completely incorporated. Sift flour and salt over mixture, and fold in.
Divide batter among liners, filling halfway. Add ganache ball to each. Cover with batter, almost to top.
Bake until just set, 8 minutes. Transfer tin to rack to cool for 10 minutes (cakes will fall). With a small offset spatula, gently remove from tin. Serve warm.
Since these are served warm, they should be baked just before serving; you can make the ganache balls three days ahead and freeze. Garnish with edible gold leaf, and serve with vanilla ice cream.
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