Preheat and Prep

Preheat your oven to 350°F (175°C).

Lightly grease a 9×13-inch baking dish with non-stick spray (preferably one that contains flour).

Make the Cake Batter

In a large mixing bowl (or stand mixer), add the flour, granulated sugar, baking powder, eggs, vanilla, and milk.

Begin mixing, then slowly pour in the melted butter while continuing to mix.

Mix just until the ingredients are combined—do not overmix, as it may toughen the cake.

Pour the batter evenly into the prepared baking dish.

Make the Cinnamon Filling

In a separate bowl, combine the softened butter, brown sugar, flour, and cinnamon. Stir until a smooth, spreadable paste forms.

Swirl the Filling

Drop spoonfuls of the cinnamon mixture evenly over the cake batter.

Use a butter knife to swirl the cinnamon mixture throughout the batter, creating a marble-like pattern. Be sure to drag the knife through both layers to incorporate.

Bake the Cake

Place the pan in the oven and bake for 35–40 minutes, or until a knife inserted in the center comes out clean.

Make the Glaze

While the cake is baking, prepare the glaze: in a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth.

If too thick, add a little more milk. If too thin, add a bit more powdered sugar.

Glaze the Cake

Once the cake comes out of the oven, pour the glaze over the warm cake.

Let the glaze sink into the swirls and edges. Allow the cake to cool slightly before slicing.