Instructions:
1. Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan and set aside.

2. In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.

3. In a separate bowl, whisk together the milk, eggs, and vanilla extract. Add the melted butter and mix well.

4. Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix.

5. In a small bowl, mix together the softened butter, brown sugar, flour, and ground cinnamon to make the cinnamon swirl.

6. Pour half of the cake batter into the prepared baking pan. Spread it evenly using a spatula.

7. Drop spoonfuls of the cinnamon swirl mixture onto the batter in the pan. Use a knife to gently swirl the cinnamon mixture into the batter.

8. Pour the remaining cake batter over the cinnamon swirl and smooth the top with a spatula.

9. Bake in the preheated oven for 35-40 minutes or until a toothpick inserted into the center comes out clean.