Martha tints Swiss meringue buttercream a golden yellow, then pipes it into petals atop moist vanilla-bean cupcakes. The best part? Candy-coated sunflower seeds form the flower centers.
Preheat oven to 350 degrees with rack in center. Line two standard muffin tins with 18 cupcake liners. In a medium bowl, whisk together flours, baking powder, baking soda, and salt.
In the bowl of an electric mixer fitted with the paddle attachment, beat together butter and sugar on medium speed until fluffy. Beat in vanilla seeds. 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.
Combine egg whites, sugar, and salt in the heatproof bowl of a mixer set over (not in) a pot of simmering water. Whisk constantly until sugar is dissolved, and mixture is warm to the touch and feels completely smooth when rubbed between fingertips.
Transfer bowl to mixer fitted with the whisk attachment. Starting on low speed and gradually increasing to medium-high, whisk until stiff (but not dry) peaks form. Continue whisking until mixture is fluffy and glossy, and completely cool (test by touching the bottom of the bowl), 6 to 8 minutes.
Reduce speed to medium-low and add butter a few tablespoons at a time, whisking well after each addition. Whisk in vanilla. Switch to paddle attachment and beat on low speed until all air bubbles are eliminated, about 2 minutes. Scrape down sides of bowl with a flexible spatula; continue beating until frosting is completely smooth. Keep buttercream at room temperature if using same day. If not, transfer to an airtight container and refrigerate up to 3 days or freeze up to 1 month. Before using, bring to room temperature and beat with paddle attachment on low speed until smooth again, about 5 minutes.
Reserve 1/2 cup buttercream. Tint remaining buttercream yellow, adding food color a drop at a time until desired color is reached. If necessary, use reserved buttercream to lighten color.
Transfer buttercream to a pastry bag fitted with a small leaf tip (such as Wilton #70). Pipe a row of slightly overlapping petals around the edge of a cupcake. Pipe a second row of petals, overlapping previous petals. Repeat piping process, overlapping petals slightly and creating 2 to 3 more rows of petals. Pipe a small amount of frosting in center of cupcake; cover with sunflower seeds. Repeat with remaining cupcakes. Decorated cupcakes can be refrigerated in an airtight container up to 6 hours; bring to room temperature before serving.