Bursting with bright citrusy flavor, these cupcakes call for orange zest in the batter and homemade orange curd in the filling. Swirls of vanilla buttercream crowned with store-bought dried orange slices take them over the top.
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.
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.
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.
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).
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.
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.
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.
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.