Preheat oven to 350F. Line standard muffin tins with paper liners. Sift together both flours, baking soda, baking powder and salt.

With an electric mixer on medium-high speed, cream butter and sugar until pale and fluffy. Reduce speed to medium. Add whole eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed. Add yolks, and beat until thoroughly combined. Reduce speed to low. Add flour mixture in three batches, alternating with two additions of buttermilk, and beating until combined after each. Beat in vanilla.

Divide batter evenly among lined cups, filling each three-quarters full. Bake, rotating tins halfway through, until cupcakes spring back when lightly touched and a cake tester inserted in centers comes out clean, about 20 minutes. Transfer tins to wire racks to cool 10 minutes; turn out cupcakes onto racks and let cool completely. Cupcakes can be stored overnight at room temperature, or frozen up to 2 months, in airtight containers.

Combine egg whites, sugar, and salt in the heatproof bowl of a standing mixer set over a pan of simmering water. Whisk constantly by hand until mixture is warm to the touch and sugar has dissolved (the mixture should feel completely smooth when rubbed between your fingertips).

Attach the bowl to the mixer fitted with the whisk attachment. Starting on low and gradually increasing to medium-high speed, whisk until stiff (but not dry) peaks form. Continue mixing until the mixture is fluffy and glossy, and completely cool (test by touching the bottom of the bowl), about 10 minutes.

With mixer on medium-low speed, add the butter a few tablespoons at a time, mixing well after each addition. Once all butter has been added, whisk in vanilla. Switch to the paddle attachment, and continue beating on low speed until all air bubbles are eliminated, about 2 minutes. Scrape down sides of bowl with a flexible spatula, and continue beating until the frosting is completely smooth. Keep buttercream at room temperature if using the same day, or 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.

Tint buttercream green with gel-paste food color. Transfer to a pastry bag fitted with a small multi-opening tip (#233). Starting at the center and moving outward, pipe rows of grass on top of cupcakes: With tip on surface of cupcake, squeeze bag, then release while quickly pulling up to form 1/4-inch-long blades, making some slightly longer than others.

Form ladybugs: Divide marzipan in half. Working on a surface lightly dusted with cornstarch, tint one portion red and the other black with gel-paste food colors. Form a 1/2-inch ball of red marzipan for each ladybug body, and a 1/4-inch ball of black marzipan for each head. Flatten the red ball slightly into an oval shape, and press the head onto the body. Pinch off black marzipan to make tiny balls for the spots, and press onto the body. Repeat to make additional ladybugs with remaining marzipan. Place one or two ladybugs on top of each frosted cupcake, gently pressing them into the buttercream grass. Refrigerate 30 minutes to allow frosting to set. Cupcakes are best served the same day they are decorated; keep at room temperature.