Preheat oven to 350℉.
Line 9x13 baking dish with parchment paper (important for cooling and cutting later)
Cream butter and both sugars (granulated sugar and brown sugar) with a mixer (standing or handheld) until completely smooth.
Mix flour, baking powder, baking soda, cream of tartar and salt into a separate bowl and set aside.
Scrape sides of the mixing bowl and then add 1 egg at a time to butter/sugar mixture while mixing on low.
Add vanilla extract and milk, and mix until fluffy.
Fold in dry ingredients with a spatula (adding a little bit at a time) until thoroughly combined.
Pour dough into parchment lined baking dish and use spatula to even out.
Mix topping cinnamon and sugar and sprinkle on top of batter
Bake for 25 minutes, or until done. Test for doneness by inserting a clean toothpick into the center - toothpick should come out clean if it is done.
Let snickerdoodle bar cool for 5-10 minutes in the baking dish, then lift the sides of the parchment paper to remove it and set aside. Let it sit and cool for another 5 minutes, then slice into smaller squares.
Serve and enjoy!