Preheat oven to 375°F.
Lightly grease an 8x8" baking dish or similar-sized casserole, or pie dish. (Make sure it is deep as well to avoid spillage while baking).
Add the strawberries, blackberries, blueberries, cherries, cornstarch, flour, lemon juice, and sugar directly to the prepared baking dish.
Toss gently until evenly coated and spread into an even layer.
In a medium mixing bowl, whisk together the flour, sugar, lemon zest, baking powder, cinnamon, and salt.
In a separate bowl, whisk together the buttermilk, melted butter, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients.
Stir gently until just combined. Do not overmix.
Spoon the batter evenly over the berry mixture and gently spread slightly if needed, but do not press the batter down into the fruit.
Sprinkle with coarse sugar if desired.
Bake for 40–45 minutes or until the topping is golden brown and the berry filling is bubbling around the edges.
If the top browns too quickly, loosely tent with foil during the final minutes of baking.
Allow the spoon cake to cool for 15–20 minutes before serving.
Serve warm with vanilla ice cream, whipped cream, or extra fresh berries if desired.