Preheat your oven to 350°F (175°C). Grease and flour a 9×9-inch baking pan.

In a medium bowl, whisk together flour, baking powder, and salt. Set aside.

In a large bowl, cream together the butter and sugar until light and fluffy. Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the butter mixture, alternating with the milk. Mix until just combined.

In a small bowl, mix together the cinnamon and sugar for the filling. Set aside.

Pour half of the cake batter into the prepared baking pan and spread it out evenly. Sprinkle the cinnamon-sugar mixture over the batter.

Spoon the remaining cake batter over the cinnamon layer and gently spread it out to cover.

For the streusel topping, combine the flour, sugar, cinnamon, and cold butter in a small bowl. Use a pastry cutter or your fingers to work the butter into the dry ingredients until crumbly. Sprinkle this topping over the cake batter.

Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely. Slice and serve.