Preheat the oven to 350°F. Grease and line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
In a heatproof bowl set over simmering water, melt the 1 cup unsalted butter and chopped 8 ounces dark chocolate together until smooth. Let cool for about 5 minutes.
1 cup unsalted butter, 8 ounces dark chocolate
Whisk the 2 cups granulated sugar, 4 large eggs, and 2 tsp vanilla extract into the melted chocolate mixture until glossy and well combined. Sift in the ¾ cup cocoa powder, 1 cup all-purpose flour, and 1 1/2 tsp salt. Fold gently until just combined, then fold in the 3/4 cup semi-sweet chocolate chips. Do not overmix.
2 cups granulated sugar, 4 large eggs, 2 tsp vanilla extract, ¾ cup cocoa powder, 1 cup all-purpose flour, 1 1/2 tsp salt, 3/4 cup semi-sweet chocolate chips
Spread the batter evenly into the prepared pan. Bake for 32–36 minutes, or until the edges are set and the center still has a slight wobble. Allow the brownies to cool completely in the pan.
Place the chopped 8 ounces semi-sweet chocolate in a heatproof bowl. Heat the 1 cup heavy cream and 1 tbsp salted butter until it just begins to simmer. Pour over the chocolate and let sit for 2 minutes. Whisk until smooth. Cover and refrigerate for 1 hour, stirring every 20–30 minutes, until thickened to a soft pudding consistency.
8 ounces semi-sweet chocolate, 1 cup heavy cream, 1 tbsp salted butter
Using a hand mixer or stand mixer, whip the chilled ganache on medium speed for 2–3 minutes until light, fluffy, and pipeable. Be careful not to overwhip. Transfer to a piping bag fitted with a large star tip.
Lift the cooled brownies from the pan and cut into 16 squares. Pipe a generous swirl of whipped chocolate ganache onto each brownie. Finish with a sprinkle of chocolate shavings, mini chocolate chips, or flaky sea salt if desired.