- 4 ounces (120 grams) white chocolate, roughly chopped
- 2 tablespoons unsalted butter
- 3 tablespoons heavy whipping cream
- 1/8 teaspoon salt
- 1 tablespoon matcha (green tea powder)
- 5½ tablespoons (2¾ ounces/77 grams) unsalted butter, plus more for greasing the ramekins
- 1 tablespoon all-purpose flour, for flouring the ramekins
- 5½ ounces (165 grams) bittersweet chocolate, chopped
- 4 large eggs, separated, at room temperature
- ¼ teaspoon salt
- 3 tablespoons sugar
- 2 teaspoons cornstarch
- To make the truffies: Combine the white chocolate, butter, cream, and salt in a double boiler or in a heatproof bowl set over a saucepan of gently simmering water and heat, stirring occasionally, until the chocolate and butter are completely melted and smooth. Whisk in the matcha until fully incorporated. Transfer the ganache to a shallow dish and refrigerate, uncovered, until firm.
- When the ganache is almost set, make the chocolate cakes: Preheat the oven to 425°F. Generously butter and flour eight 4-ounce ramekins and set aside.
- Melt the butter and chocolate in a double boiler or in a heatproof bowl set over a saucepan of gently simmering water, stirring occasionally, until completely melted and smooth. Remove from the heat.
- Put the yolks, salt, and half the sugar in a large mixing bowl and whisk just until blended. Add the melted chocolate mixture, whisk well, and set aside.
- Put the egg whites in the bowl of an electric mixer fitted with the whisk attachment and whisk at medium-high speed until frothy. With the machine running, slowly add the remaining sugar and continue whisking until medium peaks form. (When you lift the whisk from the mixture, a peak will form and the very tip of the peak will fall back down.) Whisk in the cornstarch until incorporated.
- Whisk half the whites into the yolk mixture until fully incorporated, then gently fold in the remaining whites. Divide the batter evenly among the ramekins.
- Use a 1-inch diameter ice cream scoop or a measuring spoon to scoop the white chocolate mixture into eight 1-inch balls. Press a ball into the center of each ramekin and use the back of a spoon to smooth the batter over the truffle until it is covered.
- Bake until the cakes are a dark chocolatey brown and dry to the touch, about 7 minutes; they should have risen about ½ inch above the rim of the ramekins. Cool for 1 minute in the ramekins, then invert onto serving plates and serve.