Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9x13-inch pan or line it with parchment paper.

Make Cinnamon-Sugar Mixture: In a small bowl, mix together 1/4 cup sugar and 2 tsp cinnamon. Set aside.

Prepare the Dough:

In a large mixing bowl, cream the butter and sugar together until light and fluffy.

Beat in the eggs, one at a time, followed by the vanilla extract.

In another bowl, whisk together the flour, cream of tartar, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients and mix until just combined.

Layer the Dough:

Divide the dough into two equal portions.

Press half of the dough evenly into the prepared pan.

Sprinkle half of the cinnamon-sugar mixture evenly over the dough.

Add the Top Layer:

Spread the remaining dough over the cinnamon-sugar layer. This is easiest if you drop small dollops of dough across the surface and gently press them together with your hands or a spatula.

Sprinkle the remaining cinnamon-sugar mixture evenly on top.

Bake:

Bake in the preheated oven for 25-30 minutes, or until the bars are golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs.

Cool and Slice:

Allow the bars to cool completely in the pan before slicing into squares or rectangles.