Begin by preheating the oven to 350°F (175°C). Line a 9x9-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal after baking.

In a large mixing bowl, combine the contents of the fudge brownie mix package with water, canola oil, and eggs. Stir until the mixture is fully blended and smooth, ensuring no dry patches remain.

Pour the prepared batter into the lined baking pan and spread evenly. Place the pan in the preheated oven and bake according to the time recommended on the brownie mix package, typically 30 to 35 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs.

Remove the brownies from the oven and allow them to cool completely in the pan on a wire rack before applying the frosting. This step is essential to prevent the frosting from melting or becoming too thin.

In a small saucepan set over low heat, melt the unsweetened chocolate and butter together. Stir continuously until smooth and fully combined, then remove from heat and set aside to cool slightly.

In a medium mixing bowl, whisk together the powdered sugar, half and half, Irish whiskey, and vanilla extract. Gradually incorporate the cooled chocolate-butter mixture, whisking until the frosting is smooth, glossy, and spreadable.

Once the brownies have cooled completely, pour the frosting over the top. Use a spatula to spread the frosting evenly. Allow it to set for at least 30 minutes at room temperature or chill slightly for cleaner slicing.