Preheat oven to 350°F (175°C). Grease and flour a 9" x 13" baking pan.
Whisk together 3 cups (360g) flour, baking powder, and salt. Set aside.
Using an electric mixer on medium speed, beat the butter, brown sugar, and 1 cup (200g) granulated sugar until creamy. Add the eggs, 1 at a time, mixing just until blended after each addition. Stir in the vanilla.
Reduce mixer speed to low and gradually add the flour mixture. Mix just until blended.
Spread about 3/4 of the batter into the prepared pan. Sprinkle with blackberries.
In a separate bowl, mix 1/2 cup (100g) granulated sugar and 1/4 cup (30g) flour. Add the peaches and bourbon, and stir until the peaches are coated. Spoon evenly over the blackberries.
Stir the pecans into the reserved batter. Dollop the batter over the peaches.
Bake for 1 hour, or until the topping is golden and the cobbler is bubbly. Cool completely on a wire rack before serving.
Garnish, if desired, with whipped cream and more glazed pecans.