Add some mint leaves to a cocktail shaker and muddle briefly. Add the bourbon, peach puree, Simple Syrup, spiced whisky and lemon juice to the shaker, add large ice and shake vigorously. Add the champagne and tumble roll once to mix the champagne throughout. Strain over fresh ice in a highball glass, garnish with a peach slice and serve.
Combine the sugar and 1 cup water in a small saucepan and bring to a boil. Cook, stirring, until the sugar dissolves. Remove from the heat and let cool completely. Keep covered in the refrigerator up to 1 month.