Preheat oven to 350°F. Grease or line an 8x8-inch baking pan.
In a large bowl, whisk together flour, sugar, baking powder, salt, and cinnamon. In another bowl, combine milk, melted butter, egg, and vanilla.
Pour the wet ingredients into the dry and mix until just combined. Fold in diced apples.
Spread the batter evenly in your pan. In a small bowl, combine flour, brown sugar, and cinnamon for the crumb. Cut in the cold butter with a fork (or your fingers) until it’s crumbly. Sprinkle evenly over the cake batter.
Bake for 35–40 minutes, or until a toothpick inserted in the center comes out clean and the crumb topping is golden brown.
Let cool for 15–20 minutes before slicing. Serve warm or at room temp—amazing with coffee or ice cream.