Ingredients
1/2 cup unbleached all-purpose flour
1/2 cup packed light-brown sugar
1/2 teaspoon kosher salt
1/2 stick cold unsalted butter (4 tablespoons), cut into pieces
1 teaspoon ground cinnamon
2 tablespoons granulated sugar
1 1/2 sticks unsalted butter, melted and cooled, plus more for dish
1 1/2 cups unbleached all-purpose flour
1/4 teaspoon baking soda
1/2 teaspoon cream of tartar
1/2 teaspoon kosher salt
1/2 cup packed light-brown sugar
1/2 cup granulated sugar
3 large eggs, room temperature
Preparation
Streusel: Whisk together flour, brown sugar, and salt. Using your hands or a pastry cutter, cut in butter until small to medium clumps form. Refrigerate until ready to use.
Topping: Stir together cinnamon and granulated sugar; set aside.
Bars: Preheat oven to 350 degrees. Butter an 8-inch square baking dish and line with parchment, leaving a 2-inch overhang on two sides. Butter parchment. Whisk together flour, baking soda, cream of tartar, and salt.
In a separate bowl, whisk together butter, both sugars, and eggs. Add flour mixture; stir to combine. Spread half of batter into bottom of dish. Sprinkle with half of cinnamon sugar. Dollop remaining batter on top; spread evenly with an offset spatula or the back of a spoon. Sprinkle evenly with streusel, then remaining cinnamon sugar. Bake until a tester inserted in middle comes out clean, 30 to 35 minutes. Let cool completely. Transfer to a cutting board using parchment overhang and cut into bars to serve, or store in an airtight container at room temperature up to 3 days.
