Ingredients
4 tablespoons unsalted butter, room temperature, plus more for pan
3/4 cup unbleached all-purpose flour, plus more for pan
3/4 teaspoon baking powder
1/4 teaspoon kosher salt
1/2 cup sugar
1 large egg, room temperature
1/4 cup whole milk
1/4 teaspoon pure vanilla extract
1 cup Swiss Meringue Buttercream
4 ounces green nonpareils
Preparation
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.
