Preheat the oven to 350°F (180°C). Grease a 9×9 pan and line with parchment paper. Set aside.

In a bowl, combine the butter, sugars, eggs, corn syrup, and vanilla until combined and smooth.

Add the flour, black cocoa powder, corn starch, baking soda, and salt to the bowl and fold together until a thick batter forms.

Fold in the milk until completely combined, then pour the batter into the prepared pan and spread smooth.

Bake for 20–23 minutes or until the internal temperature reaches at least 165°F (74°C). Allow the brownies to cool completely prior to removing from the pan and cutting into squares.

To decorate with a ghost, spoon a small dollop of melted white candy melts onto the top of a brownie square and then use the back of the spoon to drag the chocolate in a downward motion. Allow the chocolate to cool completely before using a very sharp knife to make the eyes and mouth by gently setting the tip of the knife into the chocolate and rotating to create the hole.

Sprinkle brownies with powdered sugar, if desired. Store at room temperature for up to 3 days.