Preheat oven to 350 degrees. Line three 12-cup mini muffin pans with foil liners. Combine confectioners' sugar, both flours, and baking powder in a medium bowl.
Heat white chocolate, butter, and soy milk in a heatproof bowl set over a saucepan of simmering water, just until melted. Let cool.
Place eggs in a mixer bowl, and whisk until thick and foamy. Slowly fold white-chocolate mixture into eggs. Fold in dry ingredients, and then vanilla-bean seeds, vanilla extract, yuzu juice, and zest.
Divide batter among prepared muffin tins. Bake until golden brown, about 16 minutes. Let cool in pan 10 minutes, then remove cupcakes to a wire rack to cool completely. Keep at room temperature up to 2 days.
Shortly before serving, spread vanilla cream onto cupcakes. Dust with silver-leaf sprinkles. Serve.
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