Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan generously. Having all ingredients at room temperature ensures optimal mixing and consistent baking results.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In a separate bowl, combine the milk, eggs, and vanilla extract. Add the melted butter to the wet ingredients and whisk until well incorporated.
Pour the wet ingredients into the dry ingredients and mix until just combined—stop when no dry spots remain to avoid overmixing.
In a medium bowl, beat the softened butter until creamy. Add the brown sugar, flour, cinnamon, and vanilla extract. Mix until a smooth paste forms, about 1-2 minutes.
Pour the cake batter into your prepared pan, spreading it evenly.
Drop spoonfuls of the cinnamon mixture over the batter. Using a knife or skewer, swirl the cinnamon mixture through the batter creating a marbled effect. Don't overswirl—you want distinct ribbons of cinnamon throughout the cake.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean with a few moist crumbs.
Allow the cake to cool for about 10 minutes. Meanwhile, whisk together the powdered sugar, milk, vanilla extract, and salt until smooth for the glaze.
Pour the glaze evenly over the still-warm cake, allowing it to seep slightly into the top layer.