Ingredients:

  • 3/4 cup unsalted butter (170g)
  • 1 cup light brown sugar, packed (200g)
  • 1 large egg, room temperature
  • 1 tsp vanilla extract (5ml)
  • 1 1/2 cups all-purpose flour (190g)
  • 1 tsp cream of tartar (3g)
  • 1/2 tsp baking soda (3g)
  • 1/4 tsp salt (1.5g)
  • 1 tsp ground cinnamon (2.6g)
  • 1 tbsp melted butter (14g)
  • 2 tbsp granulated sugar (25g)
  • 1 tsp ground cinnamon (2.6g)

Instructions:

  1. Preheat your oven to 350°F (175°C). Line your 8x8 pan with parchment paper, leaving an overhang on the sides for easy removal.
  2. Melt the butter in a saucepan over medium heat. Continue cooking, whisking constantly, until the butter foams and brown bits form at the bottom. Once it smells nutty and looks golden brown, remove it from the heat immediately to prevent burning.
  3. Let the brown butter cool slightly, then stir the brown sugar into the butter until combined. Add the egg and vanilla, whisking until the batter looks glossy and smooth. Fold in the flour, cream of tartar, baking soda, salt, and cinnamon using a spatula. Mix just until no streaks of flour remain—do not overmix.
  4. Spread the batter evenly into the prepared pan. In a small bowl, combine the topping ingredients into a paste. Dollop the cinnamon sugar over the batter and use a knife to swirl it gently into the top layer.
  5. Bake for 22–25 minutes. The edges should be set and golden, but the center should still have a slight jiggle. Remove from the oven when the top is a matte mahogany color. Let them cool completely in the pan to ensure the center sets into a fudgy consistency.