1/4 cup heavy cream
6 ounces chopped dark chocolate, divided (about 1 cup)
4 tablespoons unsalted butter, cubed
3 large eggs
1/2 teaspoon pure vanilla extract
1/4 cup granulated sugar
1/4 cup all-purpose flour
Powdered sugar, for serving
Spray four 4-oz ramekins with baking spray with flour. Or, butter the entire inner surface of the ramekin, roll cocoa powder around in it to evenly coat the side and bottom, then invert and lightly tap the bottom to remove excess.
Place 2 oz. of the chocolate in a small bowl. Heat cream and a pinch of salt in a small saucepan over high. As soon as cream begins to boil, pour over chocolate and let stand 3 minutes. Whisk with a fork until melted and smooth. Refrigerate while you prepare the batter, about 30 minutes.
Heat oven to 375°F. Melt remaining 4 oz. chocolate (about 3/4 cup) and butter in a large bowl set over a saucepan of simmering water, stirring often, 3 to 4 minutes. Remove from heat and let cool.
Combine eggs, vanilla, and sugar in a large bowl or bowl of a stand mixer. Beat at medium-high speed until pale in color and doubled in size, 5 to 7 minutes.
Pour egg mixture over melted chocolate and stir to combine. Add flour and 1/4 teaspoon salt; fold with a rubber spatula until just combined.
Divide half the batter among prepared ramekins. Place 1 heaping teaspoon ganache in center of each. Divide remaining batter among ramekins. Bake until puffed and tops are firm, about 15 minutes. Let cool 2 minutes, then run a knife around the outside of each cake and invert onto plates. Dust with powdered sugar.
Variation 1: Replace ganache with 1/4 oz. caramels and a pinch of flaky salt in each cake.
Variation 2: Replace ganache with 1 teaspoon creamy peanut butter in each cake.