Preheat your oven to 350°F (175°C). Grease and line your 9x13 inch baking pan with parchment paper for easy removal.
In a double boiler or a microwave-safe bowl, combine the unsalted butter and unsweetened chocolate. Melt them together until smooth, stirring occasionally. Allow it to cool slightly.
In a large mixing bowl, combine the melted chocolate mixture, vegetable oil, granulated sugar, light Karo syrup, eggs, and vanilla extract. Whisk until well combined.
In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture until just combined, being careful not to overmix.
Gently fold in the chopped candy and semi-sweet or milk chocolate chips, ensuring they are evenly distributed throughout the batter.
Pour the batter into the prepared baking pan and spread it evenly. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).
While the cake bakes, prepare the ganache. In a small saucepan, combine the unsalted butter, heavy whipping cream, light Karo syrup, and vanilla extract. Heat over medium heat until the mixture is steaming. Remove from heat and stir in the chocolate chips until melted and smooth.
Once the cake is done baking, allow it to cool in the pan for about 10 minutes before transferring it to a cooling rack. Once completely cool, drizzle the chocolate ganache over the top and sprinkle with additional chopped candy for garnish.
Once garnished, slice the Gooey Brownie Cake into squares and serve. Enjoy the ooey-gooey goodness!