Preheat the oven to 350°F (175°C)
Grease and flour a baking pan
Mix flour, sugar, baking powder, and salt in a bowl
Cut in butter until the mixture becomes crumbly
Whisk milk, eggs, and vanilla in a separate bowl
Combine the wet and dry ingredients until just mixed
Pour half of the batter into the prepared pan
Sprinkle cinnamon sugar or streusel over the batter
Add the remaining batter on top
Top with more streusel if desired
Bake until a toothpick inserted in the center comes out clean
Let the coffee cake cool before slicing