Preheat the oven to 350°F. Grease a 9x13-inch baking dish and set aside.
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.
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 batter and going up and down the length of the pan.
Bake for 28–32 minutes.
Mix all glaze ingredients in a small bowl. Drizzle over the cake while it is still warm.