Preheat the oven to 350°F. Line an 8x8-inch pan with parchment paper.
In a large mixing bowl, combine the melted butter, brown sugar, and granulated sugar. Stir well to combine. Add the egg and vanilla, mixing again until fully incorporated. Stir in the flour, baking soda, and salt until only a few streaks of flour remain. Stir in the chocolate chips. Set aside.
5 Tablespoons salted butter, ½ cup (100g) light brown sugar, ¼ cup (50g) granulated sugar, 1 large egg, 1 teaspoon pure vanilla extract, 1 cup (141g) all-purpose flour, ½ teaspoon baking soda, Pinch of salt, 1 cup semisweet chocolate chips
In another large bowl, melt the butter and chopped chocolate in the microwave in 20 second bursts of heat, stirring well after each interval until melted and smooth. Once melted, add the brown sugar and eggs. Mix well. Stir in the flour, baking powder, and cocoa powder by hand just until combined. Do not overmix.
½ cup salted butter, 5 ounces dark chocolate, 1 cup (200g) light brown sugar, 2 large eggs, 1 teaspoon pure vanilla extract, ½ cup (70g) all-purpose flour, ½ teaspoon baking powder, 2 Tablespoons unsweetened cocoa powder
Pour about 1/2 of the brownie batter into prepared pan. Drop spoonfuls of cookie dough randomly over the brownie batter, alternating with spoonfuls of the remaining brownie batter.
Bake for 25-30 minutes or until a toothpick inserted into the center of the brownie layer comes out mostly clean. It is better to underbake than overbake these bars.