Preheat the oven to 325°F and line an 8x8-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal.

To a saucepan over low heat, add the butter and heat until melted. Then add in the 6 ounces of chopped bittersweet chocolate, stirring frequently, until melted, then turn off the heat.

Whisk the sugar into the warm butter/chocolate mixture for 1 to 2 minutes, until the mixture is glossy and well combined. Whisk in the eggs one at a time, followed by the egg yolk and vanilla extract, mixing until the batter is smooth and slightly thickened.

To a separate medium-sized mixing bowl, combine the cocoa powder, flour, and salt and whisk until evenly incorporated. Fold this dry mixture into the wet chocolate butter mixture in 3 batches until no dry streaks remain.

Pour out and spread the batter evenly into the prepared pan.

Bake for 24 to 28 minutes, or until the edges are set and a toothpick inserted into the center comes out with some moist crumbs attached.

Let the brownies cool completely in the pan.

To make the ganache, heat the heavy cream in a small saucepan over low heat until steaming, but not boiling. Pour the heavy cream over the chopped chocolate in a heat safe bowl. Let it sit for 1-2 minutes, then whisk until the chocolate is fully melted. If it's not fully melted, microwave for about 5-7 seconds until fully melted.

Pour the ganache on top of the cooled brownies and spread into an even layer. Top with sprinkles, then let the ganache set before slicing.