01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish thoroughly.
02 - In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt until evenly blended.
03 - In a separate bowl, whisk together melted butter, whole milk, eggs, and vanilla extract until fully incorporated.
04 - Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
05 - Spread the batter evenly into the prepared baking dish, smoothing the top with a spatula.
06 - In a small bowl, blend brown sugar, flour, and ground cinnamon. Stir in melted butter until mixture resembles a thick paste.
07 - Drop spoonfuls of the cinnamon mixture evenly over the batter and gently swirl it through the batter using a knife.
08 - Bake for 32 to 36 minutes, or until a toothpick inserted into the center comes out clean.
09 - As the cake cools slightly, whisk together powdered sugar, milk, and vanilla extract until smooth and pourable.
10 - Drizzle glaze generously over the warm cake. Allow to cool slightly before slicing and serving.