CAKE: In a large bowl, whisk together 3 cups of flour, granulated sugar, baking powder and salt. Make a "well" in the center of the flour mixture. Pour in 1 1/2 cups milk, eggs, and 2 teaspoons of vanilla. Beat with an electric mixer until smooth. Once well combined, slowly stir in a 1/2 cup of melted butter. Pour the batter into a well greased 9x13-inch baking pan.
TOPPING: In large bowl, mix 1 cup of softened butter, brown sugar, 2 tablespoons flour and cinnamon until well combined. Drop evenly over the cake batter, then swirl with a knife.
Bake at 350 degrees for 28-32 minutes or until toothpick inserted in the center comes out clean.
GLAZE: In a large bowl, combine confectioner's sugar, milk, and vanilla. Beat until smooth. Drizzle over warm cake. Serve warm or at room temperature.