Preheat oven to 350°F (180°C) or follow brownie mix instructions.

Line a 9x9-inch baking pan with parchment paper and spray with cooking spray.

Prepare the brownie batter according to package directions, pour into prepared pan, and bake as directed.

Once baked, set aside to cool.

In a bowl, whisk together eggs, 3/4 cup sugar, salt, and cornstarch until mixed.

In a saucepan, combine milk, heavy cream, and remaining 1/4 cup sugar. Heat over medium-low until simmering.

Remove from heat and whisk a portion of the warm milk mixture into the egg mixture to temper the eggs.

Stir in vanilla, then return the egg mixture to the saucepan, stirring over low-medium heat until pudding thickens (about 5-6 minutes).

Off the heat, whisk in butter until smooth. Optional: pass pudding through a fine mesh strainer for a silky texture.

Spread the pudding over the cooled brownies in an even layer. Cover with plastic wrap directly on the surface to prevent skin formation.

Refrigerate for 2-3 hours to set.

Slice the brownies into 12 rectangles.

Sprinkle sugar (1-2 teaspoons) on top of each brownie and caramelize using a kitchen torch for about 30 seconds.

Let cool slightly before serving.