3 cups all-purpose flour
1 cup white granulated sugar
4 tsp. baking powder
2 large eggs
2 tsp. vanilla extract
1 1/2 cups milk
1/2 cup (1 stick) salted butter, melted

3/4 cup (1 1/2 sticks) salted butter softened to room temp
1 cup packed light brown sugar
2 tbsp flour
1 tbsp ground cinnamon

2 cups powdered sugar
5 tbsp milk
1 tsp vanilla extract

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).
Don't overmix, just stir until combined.
While your mixer is running, slowly add the 1/2 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.

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.

Whisk together powdered sugar, milk and vanilla extract in small bowl.
Pour the glaze over the warm cake.