Preheat & Prep Pans: Preheat oven to 180 °C (350 °F). Grease and line a 12-cup muffin tin or mini cake pan with paper liners.

Make the Sponge Batter: In a mixing bowl, whisk 1 egg and ⅓ cup sugar until pale and slightly thickened. Add yogurt and milk, whisking until smooth. Sift in flour and baking powder, folding gently until just combined. Drizzle in melted butter and fold to incorporate without overmixing.

Bake the Cakes: Divide batter evenly among molds, filling each about ¾ full. Bake 15–20 minutes until golden and springy, or until a toothpick inserted comes out clean. Cool in the pan for 5 minutes before transferring to a wire rack.

Prepare the Orange Cream: In a saucepan, whisk egg yolk, sugar, and cornstarch until smooth. Slowly whisk in milk, then add orange zest. Cook over medium heat, stirring constantly, until thickened—about 7–10 minutes. Remove from heat and cool, stirring occasionally.

Fill the Cakes: Once cooled, use a knife or apple corer to make a cavity, or slice cakes in half. Spoon or pipe in orange cream and replace the tops.

Decorate & Serve: Dust with icing sugar and garnish with chocolate drops. Serve fresh or refrigerate up to 2 hours before serving.