For the cupcakes: Preheat the oven to 350 degrees F. Place 12 liners in a cupcake pan.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. In a large bowl, beat the sugar and butter together using an electric mixer until it looks creamy. Beat in the egg, vanilla extract and peach extract until the mixture is smooth. Add half of the flour mixture to the butter mixture and stir until almost combined. Add the buttermilk and peach puree and stir again until almost combined. Add the rest of the flour and stir until all the ingredients are mixed in.
Fill the cupcake liners with batter using a large ice cream scoop. Bake for 18 to 20 minutes. Cool for 10 minutes before removing from the pans. Place on a wire rack and cool completely.
For the filling: Place small saucepan on the stovetop and put on medium heat. Put the peaches and sugar in and bring to slight boil. In a separate small bowl, mix the cornstarch and 1 tablespoon water until no lumps. Add the cornstarch mixture to the pan and keep on medium-low heat for about 1 minute. Cool completely before use. (Makes about 1 1/2 cups filling.)
For the frosting: Put the mascarpone, cream and confectioners' sugar in a stand mixer with the paddle attachment and beat on medium speed until thick, 4 to 5 minutes. (Makes about 2 cups frosting.)
Fill the cupcakes with the peach filling and then frost with the mascarpone frosting.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.