Preheat your oven to 350°F (175°C) and line a cupcake pan with cupcake liners.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy using an electric mixer or whisk.
Add the eggs, buttermilk, and vanilla extract to the butter mixture and mix until well combined.
In another bowl, combine the flour, baking powder, baking soda, and salt. Gradually add this dry mixture to the wet ingredients while mixing until just combined.
Gently fold in the finely chopped peaches and peach nectar, being careful not to overmix.
Divide the batter evenly among the cupcake liners, filling each about two-thirds full.
Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean. Remove from the oven and allow to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
While the cupcakes cool, prepare the frosting. In a bowl, cream together the softened butter and powdered sugar. Mix in the peach nectar and champagne (if using) until smooth and fluffy.
Once the cupcakes are completely cooled, use a spatula or piping bag to frost each cupcake with the peach frosting.
Garnish with fresh peach slices before serving.