Preheat the oven to 325°F. Line the bottom and sides of an 8-inch-square baking pan with two parchment slings, leaving some overhang for easy removal. In the top of a double boiler or a heatproof bowl set over a saucepan of water at a rolling simmer, combine the butter and chocolate chips. Whisk together as they melt. Once fully melted, add the sugar, cocoa powder, and salt. Whisk until the mixture is fully combined, then remove from heat and let rest a few minutes until you can press a finger to the bowl or top of the double boiler without wanting to pull it away. Whisk in the eggs and egg yolk one at a time, mixing until fully combined after each addition. Once no streaks of egg remain, whisk in the flour, then whisk for another 30 seconds. It will feel thick and tough, and if you’re having trouble moving the whisk through the batter, switch to a silicone spatula to finish. Transfer to the prepared baking pan and smooth the batter out as best you can with a silicone spatula. Bake 22 to 26 minutes, until the top is shiny and set and the sides are very slightly puffed up, with the center quite soft to the touch. A toothpick won’t come out clean, so don’t bother. Let the pan of brownies sit away from heat for about 5 minutes, then transfer it to the freezer to cool down while you make the toppings. (Note: if you’re making the brownie layer far in advance, cool in the refrigerator, covered, for up to 3 days before removing and proceeding with the rest of the recipe.) Make the ganache: Place the chocolate chips in a heat-safe bowl. Heat the cream, stirring with a silicone spatula, in a medium saucepan until it comes to a rolling simmer. Remove from heat and pour over the chocolate chips. Leave alone for about 1 minute, then mix rapidly with the silicone spatula, beginning in the center and working toward the perimeter. Once smooth, add the salt and continue to mix. Pour onto chilled brownies and spread with an offset spatula. Decorate with candy-covered chocolate chips or sprinkles. Chill in the refrigerator for another 30 minutes or so, just to set the ganache. Slice into 25 brownies, or 10 larger two-brownie rectangles with a shallow knife cut in the center to mimic the real deal.