Preheat the oven to 350º F (180º C). Lightly grease a 10-inch (24 cm) tube or Bundt pan (12 cups capacity) with nonstick baking spray with flour.
In a large bowl, sift flour, baking powder, baking soda and cream of tartar. Add salt and mix to combine. Set aside.
In a large bowl, beat the butter until creamy and pale on medium-high speed for 3 -4 minutes. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Add cream cheese and continue mixing for 1 minute. Add sugar and mix for 3 more minutes until light and fluffy. Add sour cream and vanilla and continue beating to combine for 1 minute.
Turn the mixer to low and add the flour mixture in three batches, alternating with milk, beginning and ending with the flour mixture; beat until just combined, about 30 – 45 seconds.
Pour the batter mixture into the prepared pan. Bake for 80 - 90 minutes, or until a skewer comes out clean when poked in the center. After the first 45 minutes of baking, loosely tent the cake with aluminum foil to ensure the surface does not over-brown. IMPORTANT: This is a large, heavy cake, so don’t be alarmed if it takes longer in your oven. If it needs longer, bake longer.
Once done, remove from the oven and cool for 15 – 20 minutes inside the pan. Then remove the loaf from the pan and set it on the wire rack to cool completely. Allow cooling completely before slicing. IMPORTANT: As you can see in the photo, the pound cake might sink a little in the middle. Don't panic; it will still be delicious.