Preheat oven to 350 degrees F. Line 2 muffin tins with about 18-20 paper liners; set aside. Meanwhile, in a large bowl, beat together the cake mix, eggs, oil, peach nectar or water, brown sugar, bourbon, and freeze-dried peach powder with an electric mixer until combined and smooth, about 2 minutes. Add in the peach halves and beat for about 30 seconds to break up the peaches into pieces.
Portion the batter evenly among each muffin cup, filling about 3/4 of the way full. Bake for approx. 15-18 minutes or until a toothpick inserted near the center comes out clean or with moist crumbs. Cool completely.
For the buttercream: In the bowl of a stand mixer, cream together the butter, bourbon, and vanilla extract until creamy, about 1 minute. Add in the freeze-dried peach powder and one cup of the confectioners' sugar, beating until combined. Alternate adding in a little of the heavy whipping cream and the remaining confectioners' sugar until frosting is light and fluffy.
Pipe or spread the frosting onto the cooled cupcakes and top with a fresh peach slice, if desired.