Preheat the oven to 400° F.To make the biscuits. In a food processor, combine the flour, baking powder, and butter. Pulse until the mix clumps together to form larger pea-size balls. Drizzle in the buttermilk until the dough is "shaggy" looking. The dough will be a little dry.Turn the dough out onto a clean surface and pat into a 1-inch thick square. Cut the dough into 4 pieces. Stack the pieces on top of each other. Press down to flatten. Dust the surface with flour and roll the dough into a 1 inch thick rectangle. Cut into 12 biscuits. Transfer to a parchment-lined baking sheet and freeze 10 minutes, to chill.Remove the biscuits from the freezer and brush with melted butter and sprinkle with cinnamon sugar. Bake for 20-25 minutes, until golden on top.Meanwhile, make the filling. In a bowl, whisk together the brown sugar, bourbon (if using), lemon juice, and vanilla until the sugar dissolves. Add the peaches and toss to coat. To make the whipped cream. In the bowl of a stand mixer fitted with the whisk attachment, beat the cream, mascarpone, sugar, and vanilla on medium-high speed until soft peaks form, 2 to 3 minutes. Chill until ready to use.To assemble, slice the shortcake horizontally and spoon the peaches and some of the juices onto the open shortcake. Top with a dollop of whipped cream, then close with the other shortcake half. Enjoy!