Preheat the oven to 375°F. Spray a 10-inch cast iron or enamel skillet with oil.
In a large bowl, whisk the cornstarch, cinnamon, and salt. Toss in the mixed berries and gently mix to coat. Add honey, lemon juice, lemon zest and vanilla then transfer to a 10-inch cast iron skillet.
Bake until the berries are hot and bubbling on the edges, about 20 to 24 minutes.
Meanwhile in a medium bowl whisk the flours, sugar, baking powder, baking soda and salt.
When the berries are ready to take out of the oven, cut in the chilled butter (must be very cold) using a pastry cutter (or 2 knives) until they form pebble-sized pieces.
In a small bowl combine buttermilk and oil; mix well. Add to the dry mixture and mix until just moistened, careful not to over-mix. Carefully remove the skillet from the oven, increase the oven temperature to 400°F and spoon the batter over the berries.
Carefully remove the skillet from the oven, increase the oven temperature to 400°F and spoon the batter over the berries.
Place back in the oven and bake until the berries are bubbling and the topping is golden and cooked through, about 15 to 18 minutes.
Remove from the oven and let it cool 15 to 20 minutes before eating.