Preheat oven to 350°F. Line a 8” or 9” round baking pan with foil and spray with nonstick cooking spray.
Make the brownies: place butter and unsweetened chocolate in a large mixing bowl. Heat on high power in 30 second increments, stirring between each, until melted and smooth (1-2 minutes total). Stir in oil, sugar, and Karo Syrup. Stir in eggs and vanilla until smooth, then mix in baking powder and salt. Slowly stir in the flour until combined, then stir in the chopped candy.
Bake for 30-35 minutes or until a toothpick comes out with just a few crumbs when poked 2” from the side of the pan. Cool completely before frosting.
To make the frosting: place chocolate chips, butter, and vanilla in a large bowl. Set aside. Bring heavy whipping cream and Karo to a boil over medium heat, stirring often. Once it’s boiling, pour it over the chocolate chip mixture. Let the mixture sit for about 30-60 seconds, then whisk until smooth. Let sit at room temperature until it thickens slightly, about an hour.
Remove brownie cake to a cake plate. Pour frosting over the top, then decorate with candy. The frosting will set after several hours (chilling helps). You may have some frosting left over.
Store loosely covered for up to 3 days.