Preheat oven to 375°F (190°C). Grease and flour a 9-inch round cake pan or an 8x8 inch square pan.

In a large bowl, whisk together flour, granulated sugar, baking powder, and salt.

In a separate bowl, whisk together melted butter, beaten egg, and milk.

Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.

Spread the batter evenly into the prepared cake pan.

In a medium bowl, combine brown sugar, flour, and cinnamon for the streusel topping. Cut in the cold butter with a pastry blender or your fingers until the mixture resembles coarse crumbs.

Sprinkle the streusel topping evenly over the cake batter.

Bake for 30-40 minutes, or until a toothpick inserted into the center comes out clean.

Let the coffee cake cool in the pan for at least 10 minutes before slicing and serving. It's delicious served warm or at room temperature.