Prepare the Baking Pan: Line an 8 × 8-inch pan with parchment paper, leaving enough paper extending over the sides to lift out the brownies later. Lightly butter the parchment and set the pan aside.
Preheat the Oven for Brownies: Preheat the oven to 350°F.
Melt the Chocolate Butter: Place the chopped semisweet chocolate and butter in a large heatproof bowl over a saucepan of simmering water. Stir as the mixture melts until combined and smooth, then remove the bowl from the heat.
Add the Sugars: Whisk the granulated sugar and brown sugar into the melted chocolate mixture until incorporated.
Incorporate the Eggs: Add the eggs one at a time, whisking well after each addition.
Add Cocoa Salt: Whisk in the sifted cocoa powder and salt until evenly incorporated.
Fold in the Flour: Add the flour and gently fold it into the batter just until combined. Avoid overmixing.
Fill the Pan: Transfer the brownie batter to the prepared pan. Use a spatula or offset spatula to spread it evenly and smooth the surface.
Bake the Brownies: Bake for 30–35 minutes, until a toothpick inserted into the center comes out clean with only a few crumbs. Do not overbake, as the brownies will continue cooking slightly after they leave the oven.
Cool the Brownies: Place the pan on a wire rack and allow the brownies to cool.
Prepare the White Chocolate: Preheat the oven to 250°F. Spread the white chocolate evenly over a rimmed baking sheet.
Begin Caramelizing: Bake the white chocolate for 5 minutes, then remove the baking sheet and stir the chocolate thoroughly with a spatula. Spread it evenly across the sheet before returning it to the oven.
Continue Caramelizing: Bake for another 10 minutes, then remove and stir thoroughly again. If chocolate sticks to the spatula, use a knife to scrape it back onto the baking sheet.
Develop the Toffee Color: Continue baking and stirring the white chocolate every 10 minutes for approximately 40–45 minutes total. Watch the color carefully, as the exact time can vary. Stop when the chocolate becomes toffee-colored but is not quite as dark as peanut butter. Transfer the Caramelized Chocolate: Pour the caramelized white chocolate into a medium or large bowl and set it aside.
Heat the Cream: Place the heavy cream in a medium saucepan and warm it over medium heat until it is just barely boiling.
Combine Cream Chocolate: Remove the cream from the heat as soon as bubbles begin forming around the edges. Pour the hot cream over the caramelized white chocolate and let the mixture stand for 30 seconds.
Whisk the Ganache: Whisk the chocolate and cream together until smooth and combined. Whisk in the vanilla and pinch of salt.
Cool the Ganache: Allow the ganache to cool slightly for about 15 minutes, whisking it a few times during this period.
Top the Brownies: Pour the slightly cooled ganache over the cooled brownies. Use an offset spatula to spread it evenly across the surface if needed.
Set the Ganache: Refrigerate the brownies for at least 15 minutes, allowing the ganache to set before serving.
Slice and Serve: Once the topping has set, cut the brownies into 16 portions and serve.
