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Preheat the oven to 350°F. Lightly grease a 9" x 13" pan.
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To make the bars: Whisk together the flour, baking powder, spice(s), and salt until well blended. Set aside.
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Melt the butter in a saucepan set over medium heat. Add the brown sugar and heat, stirring constantly, until the mixture comes together and forms a fairly smooth, shiny paste, about 3 to 5 minutes. If you have a digital thermometer, the temperature will reach about 200°F; but no worries if you don't have a thermometer, it's not critical.
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Transfer the hot mixture to a medium bowl and allow it to cool for 5 minutes, stirring occasionally. Again, if you have a thermometer, the temperature of the cooled mixture should be no higher than about 160°F; you're going to add eggs next, and you don't want them to cook.
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Mix in the eggs and vanilla. Add the flour mixture and stir until well combined; the batter will be stiff. Spread the batter evenly into the prepared pan.
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Bake the bars for 17 to 21 minutes (for chewy bars) or 23 to 27 minutes (for cakier bars), until a toothpick inserted in the center comes out with a few moist crumbs clinging to it; the top crust will have risen and fallen.
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Remove the bars from the oven and allow them to cool for 15 minutes.
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To make the topping: Mix together the sugar and spice(s) in a small bowl.
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Gently brush the melted butter evenly over the surface of the warm bars, then sprinkle on the spiced sugar, shaking and tilting the pan to distribute the sugar evenly.
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Allow the bars to cool completely, then cut them into squares.