1. Heat the oven to 350°F (175°C) and line a 12-cup muffin pan with paper liners.

2. Massage the orange zest into the sugar with your fingertips until the mixture feels moist and smells strongly of orange.

3. In another bowl, whisk together the cake flour, baking powder, and salt. Add the orange sugar and softened butter. Mix on low until the mixture resembles fine crumbs with no large pieces of butter.

4. Whisk the eggs, milk, orange juice, and vanilla bean paste together in a separate cup. Add the liquid mixture to the flour mixture in two portions, beating for about 20 seconds after each addition. The finished batter should be thick and fluffy.

5. Divide the batter between the liners, filling each about two-thirds full. Bake for 16–18 minutes, or until a toothpick inserted in the center comes out clean.

6. Leave the cupcakes in the pan for 5 minutes, then move them to a wire rack and cool completely.

7. For the curd, combine the orange juice, zest, sugar, egg yolks, whole egg, and salt in a small saucepan while the pan is still off the heat. Whisk until smooth.

8. Cook over medium-low heat for 6–8 minutes, stirring constantly, until the curd thickens enough to coat the back of a spoon. For a fully cooked egg mixture, heat it to 160°F.

9. Take the saucepan off the heat and whisk in the cold butter one cube at a time until the curd becomes smooth and glossy.

10. Pass the curd through a fine mesh strainer into a clean bowl. Cover the surface directly with plastic wrap and refrigerate for at least 1 hour.

11. Once the cupcakes are completely cool, cut a small opening in the center of each one, about 1 inch wide and two-thirds of the way down.

12. Fill each cupcake with approximately 1 tablespoon of chilled orange curd. Replace the small cake pieces on top if desired.

13. For the buttercream, beat the softened butter alone for about 2 minutes until pale and smooth. Gradually add the powdered sugar while mixing on low.

14. Add the orange juice, orange zest, vanilla bean paste, and salt. Beat on medium for about 2 minutes until airy and smooth. Add heavy cream a little at a time if the frosting needs to be softer.

15. Pipe or spread the orange vanilla buttercream over each cupcake, making sure the curd is covered.