• Preheat the oven to 350°F. Spray an 8x8 pan with cooking spray and line with parchment paper. Set aside.

Make the cinnamon crumb topping.

• In a small, microwave safe bowl, melt the butter. Set aside.
  6 Tablespoons unsalted butter
• In a medium bowl, whisk together flour, both sugars, cinnamon, and salt.
  1 ⅛ cup all-purpose flour, ¼ cup dark brown sugar, 2 Tablespoons granulated sugar, 1 ½ teaspoons ground cinnamon, 1 pinch fine sea salt
• Add melted butter to the flour mixture and toss with a fork until fully moistened. Cover the crumbs and place the bowl in the fridge while you make the cake batter.

Make the cake batter.

• In a large bowl, whisk to combine flour, sugar, baking powder, baking soda, and salt.
  1 ⅛ cup all-purpose flour, ¾ cup granulated sugar, ¾ teaspoon baking powder, ¼ teaspoon baking soda, 1 pinch fine sea salt
• Cut the butter into squares and add it to the bowl with the flour mixture. Using a pastry cutter, cut the butter into the flour mixture until it's the texture of coarse meal.
  ¼ cup unsalted butter
• In the same bowl you used to melt the butter for the crumb topping, whisk together the buttermilk, egg, and vanilla until well blended.
  ½ cup buttermilk, 1 large egg, 1 teaspoon vanilla extract
• Add the buttermilk mixture to the bowl with the flour and stir just until blended.

Assemble and bake the coffee cake.

• Add half of the cake batter to the prepared pan. Use a rubber or offset spatula to spread the batter to the edges of the pan.
• Spread half of the crumb mixture over the batter in the pan, using your fingers to crumble it into small crumbs and ensuring the batter is evenly covered and the crumbs reach to the edges.
• Add the remaining cake batter on top of the crumb layer, using the spatula to gently and carefully spread the batter over the crumbs so that the crumbs are completely covered and the batter reaches to the edges of the pan. (Adding dollops of batter evenly over the top makes spreading the batter over the crumbs much easier. A small cookie scoop is useful here.)
• Spread the remaining crumb mixture over the top of the batter, again using your fingers to crumble it into small crumbs and ensuring the batter is evenly covered and the crumbs reach to the edges.
• Bake the cake for 30 to 35 minutes or until the cake is puffed and golden brown and the top springs back when gently poked.
• As the cake cools, use a strainer or sifter to dust the top with confectioners' sugar. Let the cake cool for at least 30 minutes on a wire rack before removing from the pan, slicing, and serving. Let the cake cool completely before storing.
  1 Tablespoon confectioners' sugar