Preheat the oven and prepare the baking dish:
Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan.

Make the white cake layer:
In a large bowl, whisk together the flour, milk, sugar, eggs, baking powder, vanilla extract, salt, and melted butter. Mix until smooth.

Pour the cake batter into the prepared pan:
Pour the cake batter into the greased 9×13-inch baking dish and spread it evenly.

Prepare the cinnamon layer:
In a medium bowl, mix together the softened butter, brown sugar, flour, and cinnamon until smooth and well combined.

Add the cinnamon layer to the cake batter:
Drop spoonfuls of the cinnamon mixture over the cake batter. Use a butter knife or skewer to swirl the cinnamon mixture into the cake batter, creating a marbled effect.

Bake the cake:
Bake the cake in the preheated oven for 30-40 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for about 10 minutes.

Prepare the glaze:
While the cake is cooling, make the glaze by whisking together the confectioners’ sugar, milk, and vanilla extract in a bowl until smooth.

Drizzle the glaze over the cake:
Once the cake has cooled slightly, drizzle the glaze over the top, allowing it to soak into the warm cake.