Preheat the oven to 350°F (175°C). Line a 12-cup muffin pan with cupcake liners.
In a bowl, whisk together the flour, baking powder, and salt.
Cream together the unsalted butter and granulated sugar until the mixture is light and fluffy, which should take about 2–3 minutes.
Add the eggs to the butter mixture one at a time, ensuring to beat well after each addition.
In a separate bowl, combine the sparkling grape juice, whole milk, vanilla extract, and peach puree.
Gradually alternate adding the dry ingredients and the wet mixture to the butter mixture, beginning and concluding with the dry ingredients. Mix until just combined.
Fill the cupcake liners about 2/3 full with batter. Bake for 18–20 minutes, or until a toothpick inserted in the center comes out clean. Allow the cupcakes to cool completely.
To prepare the buttercream, beat the unsalted butter until smooth. Gradually incorporate the powdered sugar, followed by the peach puree, sparkling grape juice, and a pinch of salt. Beat until the mixture is light and fluffy.
Once the cupcakes are completely cool, frost them with the buttercream using a piping bag. Garnish with fresh peach slices and sparkling sugar.
