For the peach mimosa cupcakes: Preheat the oven to 275 degrees F on the convection setting. Line a cupcake pan with 24 cupcake liners.
In a mixer with a paddle attachment, cream the butter on medium for 3 minutes. Gradually add in the sugar and mix for 2 minutes. Scrape the bowl and add all of the egg whites and mix for 2 minutes. Combine the cake flour and baking powder in a medium bowl and whisk. In another bowl, add the peach syrup to the buttermilk. Add in one-third of the dry mixture, and then add one-third of the buttermilk mixture. Continue this process until all ingredients are incorporated. Fold in the peaches. Fill the prepared cupcake pan three-quarters full with the cupcake batter. Then bake until the top springs back to the touch, about 20 minutes. Let the cupcakes cool completely.
For the meringue: Bring a small amount of water in a medium double boiler pot to a simmer. In a medium bowl, mix the sugar and egg whites with a whisk until combined. Place the bowl on top of the double boiler and whisk constantly until the temperature reads 140 degrees F on a candy thermometer. Remove from the heat and transfer to a mixer. Mix on high speed with a whisk attachment for 5 minutes. Add the cream of tartar. Mix for 2 more minutes and slowly add in the Champagne until completely combined. Mix the meringue until it is shiny and stiff peaks form.
To assemble: Top the cooled cupcakes with the meringue and torch until slightly browned.