Preheat your oven to 350°F (175°C).

Grease a 9x13 inch baking pan with butter or non-stick spray.

In a mixing bowl, whisk together 2 cups flour, 1 tablespoon baking powder, 1 teaspoon ground cinnamon, and 1/2 teaspoon salt.

In another bowl, cream 1/2 cup softened unsalted butter and 1 cup granulated sugar until light and fluffy.

Add 2 large eggs, 1 cup milk, and 1 teaspoon vanilla extract to the butter mixture and mix until combined.

Gradually add the dry ingredients to the wet mixture, stirring until just combined without overmixing.

In a small bowl, combine 1/2 cup brown sugar, 1/4 cup flour, 2 tablespoons melted butter, and 1 teaspoon ground cinnamon to make the streusel.

Pour half of the batter into the prepared pan, sprinkle half of the streusel on top, then spoon in the remaining batter and top with the rest of the streusel.

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

Let the cake cool for at least 10 minutes on a cooling rack before slicing.