Preheat your oven to 350°F (175°C). Grease a baking pan.
In a mixing bowl, combine flour, sugar, baking powder, and salt.
Add milk, egg, and melted butter to the dry ingredients. Mix until just combined.
Pour the batter into the prepared baking pan.
In another bowl, mix brown sugar, flour, and cinnamon for the crumb topping. Add melted butter and mix until crumbly.
Sprinkle the crumb topping evenly over the batter.
Bake for 30 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool before serving.