FOR THE CAKE:
3 cups all-purpose flour
1 cup white granulated sugar
4 teaspoons baking powder
2 large eggs
2 teaspoons vanilla extract
1 ½ cups milk
½ cup (1 stick) salted butter, melted
FOR THE CINNAMON FILLING:
¾ cup (1 ½ sticks) salted butter, softened to room temperature
1 cup packed light brown sugar
2 Tablespoons flour
1 Tablespoon ground cinnamon
FOR THE GLAZE:
2 cups powdered sugar
5 Tablespoons milk
1 teaspoon vanilla extract
FOR THE CAKE:
Preheat oven to 350F degrees. Spray 9 x 13 baking dish with nonstick cooking spray (I like the kind that has flour in it.) In a large bowl (or the bowl of your stand mixer), combine ingredients for cake (except melted butter).
While your mixer is running, slowly add the ½ cup melted butter. Mix just until combined (overmixing flour in baked goods can make it tough and rubbery.)
Spread this batter evenly into your baking dish.
FOR THE FILLING:
In a small bowl, combine all your filling ingredients: softened butter, brown sugar, flour and cinnamon. Stir until combined well.
Drop the cinnamon filling by spoonfuls onto the cake batter.
Take a butter knife and make swirly, marble-effect patterns all around the cake. Drag the knife through both the cinnamon and cake batter.
Bake the cake for about 35-40 minutes (it took about 40 minutes in my oven), until a knife inserted into the center comes out clean.
FOR THE GLAZE:
Whisk together powdered sugar, milk and vanilla extract in small bowl.
Pour the glaze over the warm cake.