Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs one at a time, blending well after each addition, then stir in the milk and vanilla extract.

In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually mix this dry combination into the wet ingredients until just combined.

In a medium bowl, combine the brown sugar, flour, and ground cinnamon for the streusel topping. Blend in the cold butter until the mixture resembles coarse crumbs.

Pour half of the batter into the prepared pan, then sprinkle half of the streusel topping. Repeat with the remaining batter and streusel.

Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.

Allow the coffee cake to cool in the pan for about 10 minutes before transferring it to a wire rack. Slice and serve.