Preheat your oven to 350°F (175°C). Grease a 9x9-inch pan.
Mix flour, granulated sugar, brown sugar, baking powder, baking soda, and salt in a bowl.
In another bowl, whisk together melted butter, milk, egg, and vanilla.
Gradually fold wet ingredients into the dry mixture until just combined.
Stir in chocolate chips gently, so you don’t break them (or eat them all).
Pour the batter into the prepared baking pan and smooth the top.
For the streusel, mix brown sugar, flour, and cinnamon in a small bowl.
Cut in cold butter until crumbly, then sprinkle the mixture over the batter.
Bake for 30 minutes or until a toothpick inserted in the center comes out clean.
Cool slightly before serving warm, or attack it directly from the pan.