Streusel: In a medium bowl 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.

Cake: Preheat oven to 350° F. 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 and stir to combine. Spread the batter evenly into the bottom of the dish, use an offset spatula if necessary. Sprinkle with half of the cinnamon sugar. Sprinkle evenly with streusel, then remaining cinnamon sugar on top.

Bake until a cake tester inserted in middle comes out clean, 30 to 35 minutes. Let cool completely. Remove binder clips and transfer to a cutting board using parchment overhang and cut into bars to serve, or store in an airtight container at room temperature for up to 3 days.