Make the Peach Pie Filling: Add diced peaches, sugars, lemon juice, vanilla, cinnamon, and nutmeg to a medium sauce pan over medium heat. Cook for 6 minutes. It will be quite bubbly. Stir the corn starch and water together in a small bowl and stir into the peach mixture. Cook for 1 minute, then remove from the heat immediately and transfer to a medium bowl to cool completely in the fridge.
Make the Maple Cupcakes: Preheat your oven to 350° F. In a large mixing bowl, beat the butter with an electric mixer on high speed until creamed. Add the brown sugar and mix until well creamed with the butter. Add the egg whites and mix until just combined. Add the vanilla and sour cream and mix until just combined. Add the dry ingredients slowly with the mixer running on low, and mix until just incorporated and the last streaks of flour disappear into the batter. Add the milk in the same way. The batter will be semi runny.
Bake: Line cupcake tins with papers, and fill each about 2/3 full with cupcake batter. Bake for 15-20 minutes. The tops will be set when lightly touched and a toothpick inserted into the center with moist crumbs - not wet batter. Let cupcakes cool in the pan a few minutes, then let cool completely on a cooling rack.
Make the Frosting: Cream the butter and cream cheese together in a large mixing bowl with an electric mixer, until creamy. Ad the powdered sugar, 2 cups at a time, mixing between each addition. Add the heavy cream, vanilla, spices, and salt, and mix until just combined. Adjust the amounts of powdered sugar and spices to taste.
Assemble the Cupcakes: Use a teaspoon to remove the centers of the cupcakes. Add 1-2 tsp of the peach pie filling to each cupcake, and top with the frosting. Frost your cupcakes with a butter knife, offset spatula, or piping bag and tip. See blog post for notes about the piping tip I used.If desired, puree the remaining peach pie filling using an immersion blender or food processor, and top the cupcakes with the peach puree and a fresh peach slice (see photos).
Serve & Store: Enjoy immediately! Store leftover cupcakes in an airtight container in the fridge for 4-5 days. Let sit out at room temp for a bit before serving.
Make-Ahead Tips: Peach pie filling can be made up to 3 days in advance and stored in the fridge. Cupcakes can be made a day ahead. Cool completely at room temperature and store in an airtight container at room temperature until ready to frost.