Preheat oven to 350 degrees with racks in the middle of the oven. Line 40 standard-size muffin cups with cupcake liners; set aside.
Whisk together both flours, baking powder, salt, and baking soda in a medium bowl.
Using an electric mixer fitted with the paddle attachment, beat butter and sugar on medium-high speed until light and fluffy, 4 to 6 minutes, scraping down sides of bowl as needed.
Reduce speed to medium and add eggs one at a time, beating to incorporate fully after each and scraping down sides of bowl as needed. Reduce mixer speed to low and gradually add flour mixture, alternating with buttermilk and ending with the flour. Beat in vanilla. The batter should be smooth and very thick.
Fill prepared muffin cups halfway full with batter; transfer to oven and bake until a cake tester inserted into the center comes out clean, 15 to 20 minutes. Transfer to a wire rack and let cool completely.
Using a 2-inch ice cream scoop, scoop buttercream on tops of cooled cupcakes; sprinkle buttercream with coconut, pressing to adhere. Using your thumb, make a well in the center of each scoop of buttercream; fill with chocolate eggs. Serve.