Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.

In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. Add the eggs, milk, and vanilla, and mix until combined.

Gradually mix in the all-purpose flour, baking powder, and salt until just incorporated. Be careful not to overmix; keep it light and fluffy!

In a separate bowl, combine brown sugar and ground cinnamon. Pour the melted butter into this mixture and stir until it forms a thick paste.

Pour half of the cake batter into the prepared baking dish. Spread the cinnamon swirl mixture evenly on top of it, then pour the remaining cake batter over the swirl.

Using a knife or skewer, gently swirl the batter to create a marbled effect.

Place in the oven and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

While the cake is baking, whisk together powdered sugar, milk, and vanilla until smooth.

Once the cake is done, allow it to cool for about 10 minutes before drizzling the glaze on top.