- Nonstick cooking spray for pan
For the bars:
- 2 3/4 cups unbleached all-purpose flour, spooned and leveled
- 2 teaspoons cream of tartar, sifted
- 1 1/4 teaspoons fine sea salt
- 1 teaspoon baking soda
- 1 teaspoon ground cinnamon
- 2 sticks unsalted butter, at room temperature
- 3/4 cup packed light brown sugar
- 3/4 cup granulated sugar
- 1 1/2 teaspoons vanilla extract
- 2 large eggs, at room temperature
- 1/4 cup whole milk
For the frosting:
- 1 1/2 sticks unsalted butter, at room temperature
- 1 1/2 cups powdered sugar
- 1 teaspoon ground cinnamon, plus more for dusting (optional)
- 1 tablespoon plus 1 teaspoon whole milk
- 1 teaspoon vanilla extract
- 1/4 teaspoon fine sea salt
1Position a rack to the center of the oven and preheat it to 325°F. Line a 9×13-inch light-colored metal baking pan with aluminum foil and spray it with nonstick cooking spray.
2Prepare the bars: In a medium bowl, whisk together the flour, cream of tartar, salt, baking soda, and cinnamon.
3In the bowl of an electric mixer fitted with the paddle attachment, beat the butter on medium speed until creamy. Add the brown and granulated sugar and vanilla and beat until light and fluffy, about 3 minutes.
4Beat in the eggs, 1 at a time, giving each about 30 seconds of beating to fully incorporate. Beat in the milk. Reduce the mixer speed to low and gradually beat in the flour mixture. Finish stirring the batter by hand to make sure every is incorporated. Spread the batter in the prepared pan and smooth the top.
5Bake for 25 to 30 minutes, or until the top looks puffy and beginning to turn golden. Rotate the pan 180 degrees, and while doing so, rap the pan on the oven rack until the bars deflate. Bake for 5 minutes more, or until the bars have pulled away from the sides of the pan and a toothpick inserted into the center comes out clean. The bars will still look quite soft. If they’ve puffed back up during the last minutes of baking, rap the pan on the countertop once again. Let cool completely in the pan set on a wire rack.
6Prepare the frosting: In the bowl of an electric mixer fitted with the paddle attachment, beat the butter on medium speed until creamy. Add the powdered sugar, cinnamon, milk, vanilla, and salt, and beat until smooth. Raise the mixer speed to high and beat until very light and fluffy, about 2 minutes. Spread the frosting over the cooled bars. Finish with a light dusting of cinnamon over the entire pan, if you wish.