Preheat your oven to 350°F (175°C). Grease a 9x9-inch pan.
Combine flour, sugar, baking powder, baking soda, and cinnamon in a mixing bowl.
Mix melted butter, milk, egg, and vanilla in another bowl until smooth.
Gradually fold wet ingredients into dry ingredients. Stir gently until just combined.
Add diced apples and mix until evenly distributed.
Pour batter into the prepared pan, spreading it evenly.
For the streusel, mix brown sugar, flour, and cinnamon in a small bowl.
Cut in cold butter until crumbly, then sprinkle over the batter.
Bake for 30 minutes or until a toothpick inserted in the center comes out clean.
Cool slightly before serving warm.