In a mixing bowl, combine flour, sugar, baking powder, salt, milk, eggs, and vanilla extract. Mix until smooth.
Pour the batter into a greased baking pan.
In a separate bowl, mix softened butter, brown sugar, and cinnamon. Drop spoonfuls of the cinnamon mixture over the batter and use a knife to swirl it through the cake.
Bake for 30 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool for a few minutes before serving.