Preheat the oven to 375 degress F. Butter a 13x9x2-inch baking pan. Mix the cinnamon and 3 tablespoons sugar in a small bowl. Sprinkle 2 teaspoons over the bottom of the pan and shake to evenly coat the bottom and ½-inch up the sides.
Whisk the flour, baking soda, cream of tartar, and salt in a medium bowl. Whisk the butter and remaining 1½ cups sugar in a large bowl until smooth, then whisk in the eggs until evenly incorporated. Add the flour mixture and stir with a spatula until fully incorporated.
Dollop two-thirds of the mixture evenly in the bottom of the prepared pan and gently spread into an even layer. Sprinkle the remaining cinnamon sugar evenly on top. Put the remaining batter in a resealable plastic bag and snip a ½-inch hole in one corner.
Pipe the remaining batter on top in diagonal lines, spacing the lines 1½ inches apart.
Bake until golden brown and baked through, 20 to 25 minutes. Cool completely in the pan on a wire rack. Cut into 6 strips crosswise and 8 strips lengthwise to form 48 rectangles.
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