Preheat oven to 350°F/ 180°C.
Prepare the filling: melt the butter in a large saucepan over medium heat. Add the apple slices, brown sugar, vanilla, cinnamon, nutmeg and allspice. Stir to combine.
In a small bowl, combine the lemon and cornstarch. Mix until well incorporated.
Add the cornstarch slurry to the apples and gently stir to combine.
Cook the apples, stirring occasionally, for 5 minutes, or until the apples begin to soften. Transfer apple filling to the prepared baking pan.
Make the topping: in a medium bowl, add the dry ingredients: flour, sugar, baking powder, salt, and lemon zest. Mix to combine.
Add milk, melted butter, and vanilla to dry ingredients. Whisk just until combined. Some small lumps are okay. Pour the batter over the apples and spread evenly.
Bake on the center rack for 40-50 minutes or until the topping is golden and the apple filling is bubbling around the edges. Remove from the oven and allow to cool for at least 5 minutes before serving.
Serve preferably warm, on its own or topped with vanilla custard.