Preheat the oven to 350 degrees F (175 degrees C). Coat a 9x9-inch pan with cooking spray.

Combine brownie mix, vegetable oil, water, and eggs in a large bowl. Mix with a wooden spoon until well blended. Spread into the prepared baking pan.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Let cool completely.

While brownies cool, bring heavy cream to a simmer over medium heat in a small saucepan. Remove from heat. Stir in chocolate until smooth. Whisk in cognac.

Refrigerate topping until thick enough to spread, about 1 hour. Spread topping over cooled brownies; cut into bite-sized squares.