For the shortcakes: Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper.
In a large bowl, stir the flour, brown sugar, baking powder and fine salt to combine. Add the butter and toss, separating the cubes and making sure each is fully coated with flour. Use your hands or a pastry cutter to rub the butter into the flour until the mixture resembles a coarse meal.
Make a well in the center of the bowl and add the buttermilk, egg and vanilla if using. Use a fork or small whisk to gently mix the ingredients together inside the well (focusing on breaking up the egg), then use your hands or a spatula to mix the dough until it is evenly combined.
Scoop heaping 1/4 cup-size mounds of dough onto the prepared baking sheet, staggering the biscuits to give them at least 1 1/2 inches all around. Brush the surface of each with cream and top with turbinado sugar.
Transfer to the oven and bake until golden brown, 22 to 25 minutes. Cool completely.
For filling and finishing: When ready to serve, heat a grill or grill pan until it's smoking hot. Clean and lightly oil the grill grates. Working in batches, place the peaches cut side down on the grill and cook until they have dark char marks, 2 to 3 minutes per peach. Remove the peaches from the grill and transfer to a plate. Drizzle each peach half with 1 teaspoon of the honey while it's still hot and sprinkle with flaky salt. Cut 4 of the peach halves up into thin slices.
In the bowl of an electric stand mixer, whip the cream, powdered sugar and vanilla to medium peaks.
To serve, tear or cut each shortcake in half horizontally and place on a plate. Spoon a generous spoonful of whipped cream on top, and then top with a peach half. Once plated, divide the sliced peaches among the shortcakes and serve immediately.