Preheat oven to 350 degrees, with racks in upper and lower thirds. Line two 24-cup mini muffin pans with paper liners. In a medium bowl, whisk together flour, baking powder, and salt. In a large bowl, using an electric mixer, beat together peanut butter, butter, and brown sugar on high until pale and fluffy, about 3 minutes. Beat in egg and egg yolk, scraping down bowl as needed. With mixer on low, beat in flour mixture, buttermilk, and vanilla until combined.
With the large end of a melon baller or a spoon, place 2 teaspoons batter into each muffin cup, then press a peanut-butter candy into each center until batter aligns with top edge of candy. Bake until puffed and set, about 10 minutes, rotating pans halfway through. Immediately place a piece of candy corn on top of each cupcake; let cool completely in pans on wire racks.
You can use any small decorative Halloween candies in place of the candy corn. Try ones shaped like pumpkins, skulls, or spiders and other creepy crawlers.