The number one reason for a flat and dense cake is overmixing the batter. You also want to make sure your butter is at room temperature and that you've taken it out of the fridge well in advance. When the butter is too cold or too soft it will have a big impact on the end result. In fact, make sure all your ingredients are at room temperature; milk, eggs, etc. You want all of your ingredients at the same temperature when mixing together.