Adapted from allrecipes.com, this is the perfect combination of brownie and cookie.
Ingredients:
Cookie layer:
½ cup unsalted pareve margarine, softened
½ cup brown sugar
¼ cup granulated sugar
½ tsp. pure vanilla extract
1 large egg, room temperature
1 ¼ cups unbleached all-purpose flour
½ tsp. kosher salt
½ tsp. baking soda
1 cup semisweet chocolate chips
Brownie layer:
1 cup granulated sugar
½ cup unsalted pareve margarine, melted
1 tsp. pure vanilla extract
2 large eggs
1/3 cup cocoa powder
½ cup unbleached all-purpose flour
¼ tsp. baking powder
1/8 tsp. kosher salt
Directions:
Preheat oven to 350F and line a 9×13-inch baking pan with parchment paper sprayed with cooking spray.
In the bowl of a stand mixer fitted with the paddle attachment, beat together the margarine, sugars, and vanilla until creamy.
Add the egg and beat until light and creamy, about 2 minutes.
Whisk flour, salt, and baking soda together in a bowl.
Gradually stir flour mixture into margarine mixture until dough is combined. 
Stir chocolate chips into dough.
Spread dough into the bottom of the prepared pan to cover completely.
Stir sugar, melted margarine, and vanilla together in the bowl of a stand mixer fitted with the paddle attachment.
Add eggs and beat well. Mix cocoa into margarine mixture until well combined.
Add flour, baking powder and salt and mix until just combined.
Pour brownie batter over cookie dough and spread to cover completely.
Bake for 25 minutes and cool completely before cutting into bars.
The Science of Baking 