Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch pan.

Make streusel topping: Combine sugar, flour, and cinnamon in a medium bowl. Cut in butter until mixture resembles coarse crumbs. Set aside.

Make cake: Combine flour, sugar, baking powder, and salt in a large bowl. Cut in butter until mixture resembles coarse crumbs. Whisk together egg and milk in a separate bowl; stir in vanilla.

Pour egg mixture into crumb mixture; mix until just moistened.

Spread into the prepared pan and sprinkle with streusel topping.

Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, 25 to 30 minutes.

Allow to cool before slicing.