• Flour • Light Brown Sugar • Kosher Salt • Unsalted Butter • Ground Cinnamon • Granulated Sugar
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.
Topping: In a small bowl mix together cinnamon and granulated sugar, set aside.
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.