Preheat the oven to 350°F. Grease a 9x13-inch baking dish and set aside. We usually use a metal 9x13-inch pan. If using glass, the cake may need a few extra minutes.
In a large bowl, mix flour, sugar, baking powder, salt, milk, eggs, and vanilla. Slowly stir in melted butter and pour the batter into the prepared baking dish. NOTE: Do not over-mix. Stir the batter just until combined so the cake stays soft and tender.
Cinnamon topping: In another bowl, mix softened butter, brown sugar, flour, and cinnamon. Dollop over the top of the cake mixture evenly.
Swirl your topping and cake mixture with a knife, only inserting it halfway into the cake batter and going up and down the length of the pan so the cinnamon mixture forms ribbons through the cake instead of staying only on top.
Bake for 28 to 32 minutes, or until a toothpick inserted into the cake portion near the center comes out clean or with a few moist crumbs. Avoid testing directly in the cinnamon swirl.
Mix all glaze ingredients in a small bowl. If the glaze is too thick, add a little more milk. If it is too thin, add a little more powdered sugar.
Pour the glaze over the warm cake so it settles into the cinnamon swirls. Let it cool slightly before slicing for cleaner pieces.