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1
Preheat oven to 350 degrees F. Butter a 9 by 13 inch baking pan, line it with parchment and lightly butter the parchment.
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2
In a medium bowl, stir together flour, baking powder, cinnamon, cream of tartar, and salt.
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3
In a large bowl, cream butter and sugar until light and fluffy. I did this by hand, but a hand or stand mixer will work too. Add the eggs one at a time, mixing until each is fully incorporated. Add the vanilla and beat until once again light and fluffy. Add the flour mixture and stir to combine. Stir in the walnuts until evenly distributed.
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4
Pour the batter into the prepared pan. Spread into an even layer. In a small bowl, stir together cinnamon and sugar for topping. Sprinkle the topping into an even layer over the batter. Use it all.
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5
Bake for 40-45 minutes, rotating halfway through if necessary, until a toothpick inserted in the center of the cake comes out clean or with a few moist crumbs. Cool completely in pan.
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6
Once the bars are cool, make the glaze. In a medium bowl, whisk together sugar, water, and vanilla until a thick, but spreadable glaze forms. Add more water to make it thinner or sugar to make it thicker as necessary. Immediately spread glaze evenly over bars. Cut into pieces and serve. Bars may be stored in an airtight container at room temperature for up to 3 days, or frozen, wrapped in pieces in parchment and foil and placed in a zipper bag for up to 3 months. Thaw at room temperature for about 2 hours.
