Preheat your oven to 350°F (175°C). Grease a 9x9-inch pan.
In a mixing bowl, whisk flour, sugar, baking powder, and salt.
Add milk, melted butter, and vanilla. Stir until smooth.
Pour the batter into the prepared pan and spread evenly.
In a separate bowl, combine brown sugar, flour, and cinnamon for the streusel.
Cut in cold butter using a fork or your hands until crumbly.
Sprinkle the streusel mixture evenly over the batter.
Bake for 25 minutes or until a toothpick comes out clean.
Let it cool slightly, then slice and serve.