Preheat your oven to 350°F (175°C). Lightly grease your 9x13-inch baking pan or line it with parchment paper for easy removal.
In a medium bowl, whisk together the flour, baking soda, cream of tartar, and salt. Set aside.
In a large mixing bowl, use an electric mixer to beat the butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes.
Beat in the eggs, one at a time, then stir in the vanilla extract until combined.
Mix in the apple butter until evenly distributed throughout the batter.
Gradually add the dry ingredients to the wet mixture, mixing on low speed or folding gently with a spatula until just combined. Avoid overmixing to keep the bars tender.
In a small bowl, combine the ground cinnamon and 1/4 cup granulated sugar. Sprinkle this evenly over the batter in the pan.
Spread the batter evenly in the prepared pan. Bake for 30-35 minutes or until the edges start to turn golden and a toothpick inserted in the center comes out clean or with a few moist crumbs.
Allow the bars to cool completely in the pan on a cooling rack before slicing into squares. This helps them set perfectly and hold their shape.