Preheat oven to 350 degrees with rack in center. Line jumbo muffin pans with 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 butter and sugar on medium speed until light and fluffy, about 2 minutes. With mixer running, add eggs, one at a time, beating to combine after each addition. Beat in vanilla.
With mixer on low speed, add flour mixture, alternating with buttermilk, beginning and ending with flour mixture.
Divide batter evenly between baking cups (2/3 cup batter per cup). Bake until light golden brown and a cake tester inserted in the center comes out clean, rotating pans halfway through baking, 30 to 40 minutes. Transfer pans to a wire rack to cool for 5 minutes. Remove cupcakes from pan to wire rack to cool completely.
To decorate, use the handle of a wooden spoon to create a deep hole in the center of each cupcake. Using a pastry bag fitted with a 1/3-inch round tip, fill cupcakes with buttercream. Frost each cupcake with about 2 tablespoons buttercream. Use cinnamon candies to make X's, O's, or hearts on top of cupcakes.