Preheat your oven to 350°F (175°C).
Grease a 9x13 baking dish or a large cast iron skillet.
In a mixing bowl, whisk together flour, sugar, and milk until smooth. Stir in vanilla if using.
Pour the melted butter into the prepared baking dish (don’t stir it in—trust the process).
Gently pour the batter over the butter. Do not mix.
Evenly sprinkle the frozen berries over the top.
Bake for 50-55 minutes or until golden and bubbly around the edges.
Serve warm with a big scoop of vanilla ice cream or whipped cream (or both, no judgment).