Preheat the oven to 350°F (176°C). Prepare a 9-inch square pan or 8-inch round springform pan with butter or nonstick cooking spray.
Whisk together the flour, baking powder, and salt in a medium bowl. Set aside.
In the bowl of an electric mixer, beat the butter, vanilla, and granulated sugar until creamy, about 2 minutes. Add the eggs one at a time, and mix until well incorporated.
Add the flour mixture and milk, and mix on low speed to combine.
Fold the berries and apples into the batter, mix gently with a spatula until well combined. Do not over-mix.
Pour the batter into your prepared pan.
Make the crumble topping. In a large, microwave-safe bowl, melt the butter for about 1 minute on high power.
Add the sugar to the melted butter and whisk to combine. Add the flour, optional salt, and stir to combine, gently. You want the mixture to be crumbly, not formed into a dough.
Add crumble all over the top of your cake batter.
Transfer cake to oven and bake at 350 (176 C) for 45 minutes.
Remove from oven and let it cool completely, and sit in the refrigerator for one hour before slicing into it.