Peel and slice each peach into 8 equal slices. Arrange on a paper towel-lined plate and set aside.

Position rack in center of oven and preheat oven to 350°F. Generously butter a 9" deep-dish pie plate.

In a medium bowl, whisk together flour, baking powder, and salt; set aside.

In a large bowl, use an electric mixer to blend the butter until creamy. Add the sugar and beat on medium-high speed for a couple of minutes until light and fluffy.

Turn down the mixer and slowly blend in the egg, buttermilk, and vanilla.

With the mixer on low, gradually mix in the flour mixture until just combined.

Scrape the batter into the prepared pie plate and smooth the top. Arrange the peach slices on top of the batter, lightly pressing them down.

Sprinkle the top of the cake with the sparkling sugar.

Bake for 10 minutes at 350°F. Then reduce the oven temperature to 325°F and continue baking for 50 to 60 minutes, or until a toothpick inserted into the center comes out clean.

Place the pie plate on a rack to cool. Slice into wedges and serve.