Grease and flour a 9” or 10” round cake pan; set aside. (Note: A cake pan with a removable bottom will make this project much easier!)
Using a medium saucepan, add the butter, unsweetened chocolate and ½ cup of chocolate chips. Place over medium-low heat, stirring often, until butter and chocolate have fully melted; set pan aside to cool.
In a medium bowl, whisk together the eggs, sugar and vanilla. Stir the egg mixture into the warm chocolate mixture.
In a separate bowl, sift together the flour, baking powder and salt. Stir the flour mixture into the chocolate mixture.
Stir ¼ cup of chocolate chips into the batter.
Pour batter into prepared baking pan. Bake at 350°F for 18-20 minutes, or until a toothpick inserted into the brownie comes out mostly clean.
As soon as you remove the brownie from the oven, sprinkle the remaining ¾ cup of chocolate chips evenly across top of brownie. (Tip: Leave a 1” border around the edge for your pizza “crust”.)
Let cool for 5 minutes and then use an offset spatula to smooth the melted chocolate chips evenly around the top of the brownie. Gently press chopped candy bars and other toppings into the melted chocolate. Let cool fully before slicing.