1. Preheat your oven to 350°F (175°C) and grease a 9×13 inch baking pan.
2. In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
3. Beat in the eggs, one at a time, followed by the vanilla extract.
4. In another bowl, whisk together the flour, baking powder, cinnamon, and salt.
5. Gradually add the dry ingredients to the wet mixture until just combined.
6. Pour half of the batter into the prepared baking pan and spread evenly.
7. In a small bowl, mix brown sugar and remaining cinnamon; sprinkle this mixture over the batter in the pan.
8. Add dollops of the remaining batter over the top and swirl gently with a knife to create a marble effect.
9. Pour heavy cream over the top of the batter for extra moisture.
10. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
11. Allow to cool before cutting into bars.