Preheat oven to 350°F (175°C). In a mixing bowl, combine flour, sugar, baking powder, and salt. Add milk, eggs, melted butter, and vanilla. Mix until smooth.
Make the Swirl:
In a separate bowl, mix softened butter, brown sugar, flour, and cinnamon until a paste forms.
Assemble Cake:
Pour the batter into a greased 9x13-inch baking dish. Drop spoonfuls of the cinnamon mixture over the batter and swirl using a knife.
Bake:
Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
Prepare Glaze:
Mix powdered sugar, milk, and vanilla until smooth.
Glaze and Serve:
Drizzle the glaze over the warm cake. Slice and serve.
Notes
Make Ahead: Bake the cake ahead of time and store it in an airtight container for up to 3 days. Add the glaze before serving.
Customizing the Flavor: Add a touch of nutmeg to the swirl for extra warmth. Top with chopped pecans or walnuts for added texture.
Serving Suggestions: Serve warm with a drizzle of glaze and a cup of coffee or tea for a cozy treat.
Storage Tips: Store leftovers in an airtight container at room temperature for 3 days or refrigerate for up to 1 week.