Preheat your oven to 375°F and grease a 9-inch square baking dish.
In a medium bowl, whisk together flour, baking powder, and salt.
In another bowl, cream the butter and sugar until light and fluffy.
Beat in the eggs one at a time, then stir in vanilla.
Add the dry ingredients to the wet mix alternately with the milk, starting and ending with dry.
Pour half of the batter into the prepared pan.
Mix brown sugar and cinnamon in a small bowl, then sprinkle half over the batter.
Dot with cold butter, then spread the rest of the batter on top and finish with the remaining cinnamon mixture.
Bake for 30-35 minutes or until a toothpick comes out clean.
Let it cool slightly before cutting. Eat warm or at room temp—both work like a charm.