Preheat your oven to 350°F (175°C) and grease and flour a 9-inch round cake pan.
In a medium bowl, whisk together the flour, baking powder, and salt.
In another bowl, beat the softened butter and sugar together until light and fluffy. Then add in the egg and vanilla extract, mixing until smooth.
Gradually alternate adding the dry mixture and buttermilk into the butter mixture. Mix just until combined.
Pour the batter into your prepared pan, smoothing it out evenly. Arrange the peach slices on top and sprinkle with sparkling sugar if desired.
Bake for about 50–60 minutes or until a toothpick inserted into the center comes out clean. Let cool in the pan for about 10 minutes before transferring to a wire rack.