In this whimsical decoration for Martha's special Easter cake, a basic vanilla cake is baked, crumbled and mixed with buttercream and formed into balls. The balls are then rolled in green nonpareils to resemble clipped boxwood hedges.
Preheat oven to 350°F. Butter a 9-inch round cake pan; dust with flour, tapping out excess. Whisk together flour, baking powder, and salt. Beat butter with sugar on medium speed until light and fluffy, 1 minute. Add egg; beat well. Reduce speed to low and gradually add flour mixture, beating until combined. Add milk and vanilla, scraping down sides of bowl as necessary; beat until just combined. Pour batter into prepared pan; smooth top with an offset spatula.
Bake until golden and a toothpick inserted into center comes out clean, 23 to 25 minutes. Let cake cool in pan on a wire rack 15 minutes, then turn out onto rack and let cool completely.
Crumble cake into a bowl until no large lumps remain. Add buttercream; stir until combined.
Place nonpareils in a small bowl. Using your hands, roll half of cake mixture into 1 1/4-inch balls (heaping teaspoons); working with one at a time, roll balls in nonpareils, pressing lightly to adhere.
Roll remaining cake mixture into 3/4-inch balls (scant teaspoons) and roll in nonpareils, pressing lightly to adhere. (You should have about 14 of each.) Refrigerate until firm, about 1 hour. Decorated cake balls can be refrigerated in an airtight container up to 3 days.