• Preheat the oven to 350℉. Line an 8x8 pan with parchment paper or spray it with nonstick baking spray. I prefer parchment so you can lift the bars easily once they're finished.

Make the crust

• Heat the butter in a microwave for 30 to 45 seconds, until everything is soft and mostly melted. Add the sugars, cinnamon, cream of tartar, salt, and vanilla. Whisk together until well blended.
  1/2 cup (112 g) salted butter, 1/3 cup (64 g) brown sugar, 1 tbsp (13 g) sugar, ½ tsp vanilla, 1/2 tsp cream of tartar, 1/2 tsp cinnamon, 1/4 tsp salt
• Add the flour and stir until no white streaks remain. You'll have a thick, clumpy dough. Press evenly into the bottom of the pan and bake for 15-18 minutes, or until the crust is golden and beginning to brown at the edges.
  1 ¼ cup (171 g) all purpose flour
• Cool on a wire rack while making the filling and the topping.

Make the filling

• In a medium mixing bowl, beat the softened cream cheese and sugar together until smooth. Add the egg, vanilla, and salt and beat on low until everything is mixed and smooth. Pour over the crust and spread evenly.
  1 block (8 ounces) softened cream cheese, ¼ cup sugar, 1 tsp vanilla, 1 egg, 1/8 tsp salt

Make the topping

• In a small mixing bowl, melt the butter. Then add the flour, brown sugar, cinnamon, and salt. Stir together until the mixture looks like course sand. Sprinkle in an even layer across the top of the filling. Don't press it in.
  1.5 tbsp salted butter, ¼ cup all purpose flour, ¼ cup (51 g) brown sugar, ¼ tsp cinnamon, ⅛ tsp salt
• Bake the bars for 25 to 30 minutes, just until the topping looks golden brown and there is only a slight jiggle in the center of the cheesecake. Cool on a wire rack for 1-2 hours at room temp, then 2-3 hours in the fridge.
• Once the bars have cooled completely (feel the bottom of the pan to see if it's chilled through), use the parchment paper to lift the bars out of the pan and cut into squares.