Heat oven to 325 degrees with rack in center. Line muffin pans with baking cups; set aside. In a medium bowl, sift together flour, baking powder, salt, and baking soda.

In the bowl of a heavy-duty electric mixer fitted with the paddle attachment, beat 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 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 half way through baking. Transfer pans to a wire rack to cool.

To decorate, place each color of sprinkles in its own small bowl. Frost each cupcake with about 3 tablespoons Meringue Buttercream. Gently press each frosted cupcake into sprinkles, rolling around so entire surface is coated. Arrange picnic party picks as desired.