Ingredients:

  • 3/4 cup (170g) unsalted butter, softened
  • 3/4 cup (150g) light brown sugar, packed
  • 1/4 cup (50g) granulated sugar
  • 1 large egg, room temperature
  • 1 large egg yolk, room temperature
  • 2 teaspoons pure vanilla extract
  • 2 cups (250g) all-purpose flour
  • 1 teaspoon cornstarch
  • 3/4 teaspoon baking soda
  • 1/2 teaspoon kosher salt
  • 1 1/4 cups (225g) semi-sweet chocolate chips
  • 1/2 cup (115g) unsalted butter, softened (for frosting)
  • 1 1/2 cups (180g) powdered sugar
  • 1/3 cup (30g) unsweetened cocoa powder
  • 2 tablespoons heavy cream
  • 1 pinch salt

Instructions:

  1. Preheat your oven to 325°F (163°C). Grease a 9-inch or 10-inch round cake pan thoroughly and line the bottom with a circle of parchment paper.
  2. In a stand mixer or with an electric hand mixer, beat the 3/4 cup softened butter, light brown sugar, and granulated sugar for 2-3 minutes until pale and aerated.
  3. Add the egg, egg yolk, and vanilla extract. Beat until the mixture is well combined and smooth.
  4. In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and kosher salt.
  5. Turn the mixer to low and slowly add the dry ingredients to the wet ingredients. Mix until just combined, then fold in the chocolate chips.
  6. Press the dough evenly into the prepared pan. Bake for 20 minutes, or until the edges are golden brown and the center is set but still soft.
  7. Allow the cookie cake to cool completely in the pan before removing. While cooling, beat frosting ingredients (1/2 cup butter, powdered sugar, cocoa, heavy cream, and salt) together until smooth. Pipe onto the cooled cake using a star tip.