Preheat and prepare the pan

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

Melt the butter and chocolate

• Add the butter to a saucepan over low heat and heat until melted.
• Add the chopped semisweet chocolate and stir frequently until melted, then turn off the heat.

Whisk in sugar, eggs and vanilla

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

Combine the dry ingredients

• In a separate medium mixing bowl, combine the cocoa powder, flour and salt and whisk until evenly incorporated.

Fold in the dry mixture

• Fold the dry mixture into the wet chocolate butter mixture in 3 batches until no dry streaks remain.

Bake the brownies

• Pour and spread the batter evenly into the prepared pan.
• Bake until the edges are set and a toothpick inserted into the center comes out with some moist crumbs attached.

Cool the brownies

• Let the brownies cool completely in the pan.

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 and let it sit.
• Whisk until the chocolate is fully melted; if needed, microwave for a few seconds until fully melted.

Top and set

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