Preheat oven to 325 degrees F with a rack in the center. Line jumbo muffin pans with paper baking cups; set aside. In a medium bowl, sift together flour, baking powder, salt, and baking soda; set aside. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar on medium speed until light and fluffy, 3 to 5 minutes. With mixer running, add eggs and vanilla extract; beat until combined. With mixer on low speed, add 1 cup reserved flour mixture; mix until combined. Add 1/2 cup buttermilk; combine. Alternate adding flour mixture and buttermilk, ending with flour, until all has been incorporated. Divide batter evenly among baking cups. Bake until light golden brown and a cake tester inserted into middle comes out clean, 25 to 30 minutes. Rotate pans halfway through baking. Transfer pans to a wire rack to cool. To decorate, frost each cupcake with about 3 tablespoons buttercream. Gently press each frosted cupcake into sprinkles, rolling around so that entire surface is coated. On a lightly sugared piece of parchment paper, roll out jellied-citrus slices to about 1/8-inch thick. Use 3/4 to 2-inch gum-paste petal cutters to cut petals from the jellied-citrus slices. Use a 3/4 or 1/4-inch pastry tip to cut out the centers of flowers. Use the green citrus slices and a small petal cutter to cut out leaves for the flowers. Place citrus dots in center of cupcakes, and arrange jellied petals around each center to form a flower. Arrange leaves as desired.
Combine egg whites, sugar, and vanilla in the heatproof bowl of an electric mixer. Set bowl over a pan of simmering water. Whisk constantly until sugar has dissolved and whites are hot to the touch, 5 to 7 minutes. Transfer bowl to the electric mixer. Using whisk attachment, mix on low speed, gradually increasing to high speed, until stiff, glossy peaks form, about 10 minutes. In a separate bowl, beat butter until creamy and fluffy. At low speed, add butter to egg whites, mixing until smooth. Add vanilla, and continue mixing until incorporated. Use immediately. Yield: 5 cups