Step 1

Cupcakes: Preheat oven to 350 degrees with rack in center. Line two standard muffin tins with 18 paper liners. In a medium bowl, whisk together flours, baking powder, baking soda, and salt.

Step 2

In the bowl of a mixer fitted with the paddle attachment, beat together butter and granulated sugar on medium speed until fluffy. Beat in vanilla seeds and orange zest. Add eggs and yolks, one at a time, beating until each is fully incorporated before adding the next. Reduce speed to low; gradually add flour mixture in three batches, alternating with buttermilk, beginning and ending with flour mixture and beating until combined after each addition.

Step 3

Using an ice cream scoop or large spoon, divide batter evenly among muffin cups, filling each two-thirds full. Bake, rotating pans halfway through, until cupcakes spring back when lightly pressed and a tester inserted into centers comes out clean, about 16 minutes. Transfer to a wire rack and let cool completely.

Step 4

Orange Curd: In a medium saucepan, whisk together granulated sugar, both zests, and egg yolks; whisk in citrus juices and salt. Add butter; place pan over medium-high and cook, whisking constantly, until butter has melted, mixture is thick enough to coat the back of a spoon, and small bubbles form around edge of pan, about 5 minutes (do not boil).

Step 5

Remove pan from heat while continuing to whisk. Pour curd through a fine-mesh sieve into a glass bowl. Press plastic wrap against surface of curd; refrigerate until cool, about 2 hours.

Step 6

Frosting: In the bowl of a mixer fitted with the paddle attachment, beat butter until smooth and creamy. With mixer on low speed, add 7 cups confectioners' sugar, milk, and vanilla; beat until light and fluffy. If necessary, gradually add remaining 1 cup confectioners' sugar until desired consistency is reached.

Step 7

Fill a pastry bag fitted with a 1/4-inch round tip with about 2 cups curd. Insert tip into center of each cupcake, squeezing gently to just fill. Reserve any remaining curd for another use.

Step 8

Fill another pastry bag fitted with a 1/2-inch star tip with frosting and pipe onto cupcakes. Decorate with dried orange slices, if desired.