Preheat the oven to 350°F (175°C) and grease a 9x13 inch baking pan.
In a large mixing bowl, whisk together the flour, sugar, baking powder, salt, and ground cinnamon.
Add the milk, melted butter, eggs, and vanilla extract to the dry ingredients. Mix until just combined, being careful not to overmix.
In a separate bowl, mix together the brown sugar, cinnamon, and softened butter for the filling until crumbly.
Spread half of the cake batter into the prepared baking pan. Sprinkle the cinnamon filling evenly on top of the batter.
Pour the remaining cake batter over the filling, then use a knife to gently swirl it together (don’t overdo it, just a few swirls to create a marble effect).
For the crumb topping, mix the flour, brown sugar, cinnamon, and salt together in a bowl. Stir in the melted butter until crumbly.
Sprinkle the crumb topping evenly over the batter in the pan.
Bake in the preheated oven for 30-40 minutes, or until a toothpick inserted in the center comes out clean.
Allow the coffee cake to cool for about 10 minutes before serving.