Preheat the oven to 350 degrees. Prepare a nine-by-13-inch baking pan by spraying it with cooking spray. Set it aside.
For the cake, in a mixing bowl, beat the flour, sugar, baking powder, eggs, vanilla and milk until everything is smooth and well combined.
Hand stir in the butter, and then spread the batter evenly into your prepared pan.
For the filling, in another mixing bowl, beat together all of the filling ingredients until it forms a smooth paste.
Drop the cinnamon mixture by the tablespoonful onto the cake batter, and then swirl it into the batter using a knife.
Bake the cake for 35 to 40 minutes or until a toothpick inserted in the center comes out clean.
When the cake comes out of the oven, whisk the glaze ingredients until it is smooth. If you want a thicker glaze, add a little more powdered sugar. If you want it thinner, add a little more milk.
Pour the glaze over the cake and serve warm. Store any leftovers in an airtight container.