Prepare the Peaches: Peel and slice each peach into 8 wedges. Place on a paper towel-lined plate to absorb excess moisture. Set aside.

Preheat and Prep: Preheat oven to 350°F (175°C). Generously butter a 9-inch deep-dish pie plate.

Mix Dry Ingredients: In a medium bowl, whisk together flour, baking powder, and salt. Set aside.

Cream Butter and Sugar: In a large mixing bowl, beat softened butter with an electric mixer until creamy. Add sugar and beat on medium-high until light and fluffy (about 2 minutes).

Add Wet Ingredients: Lower mixer speed and blend in the egg, then the buttermilk and vanilla, mixing until fully incorporated.

Combine Batter: Gradually add the flour mixture on low speed, mixing until just combined. Do not overmix.

Assemble the Cake: Pour batter into the greased pie plate and smooth the surface. Arrange peach slices on top, pressing gently into the batter with little space between each slice.

Add Topping and Bake: Sprinkle sparkling sugar over the top. Bake at 350°F for 10 minutes, then reduce temperature to 325°F and bake an additional 50–60 minutes, or until a toothpick inserted in the center (avoiding peaches) comes out clean.

Cool and Serve:

Let cake cool on a wire rack. Serve warm or at room temperature with whipped cream or ice cream if desired.